diff --git a/README.md b/README.md
index 36d6b62..dca4ca7 100644
--- a/README.md
+++ b/README.md
@@ -36,17 +36,20 @@ SOFTWARE.
  * `nether_portal_ignite.ogg` is a derivative of "Flame Ignition" by [hykenfreak](https://freesound.org/people/hykenfreak), used under CC BY 3.0. "Nether Portal ignite" is licensed under CC BY 3.0 by Treer.
 
 ### [Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)](https://creativecommons.org/licenses/by-sa/4.0/)
+ * `nether_basalt`* (files starting with "nether_basalt"): Treer, 2020
  * `nether_book_`* (files starting with "nether_book"): Treer, 2019-2020
  * `nether_fumarole.ogg`: Treer, 2020
+ * `nether_lava_bubble`* (files starting with "nether_lava_bubble"): Treer, 2020
+ * `nether_lava_crust_animated.png`: Treer, 2019-2020
  * `nether_particle_anim`* (files starting with "nether_particle_anim"): Treer, 2019
  * `nether_portal_ignition_failure.ogg`: Treer, 2019
  * `nether_smoke_puff.png`: Treer, 2020
 
 ### [Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)](http://creativecommons.org/licenses/by-sa/3.0/)
- * `nether_glowstone.png`: BlockMen
+ * `nether_glowstone`* (files starting with "nether_glowstone"): BlockMen
  * `nether_nether_ingot.png` & `nether_nether_lump.png`: color adjusted versions from "[default](https://github.com/minetest/minetest_game/tree/master/mods/default)" mod, originally by Gambit
  * `nether_portal.png`: [Extex101](https://github.com/Extex101), 2020
- * `nether_rack.png`: Zeg9
+ * `nether_rack`* (files starting with "nether_rack"): Zeg9
  * `nether_tool_`* (files starting with "nether_tool_"): color adjusted versions from "[default](https://github.com/minetest/minetest_game/tree/master/mods/default)" mod, originals by BlockMen
 
 All other media: Copyright © 2013 PilzAdam, licensed under CC BY-SA 3.0 by PilzAdam.
\ No newline at end of file
diff --git a/crafts.lua b/crafts.lua
new file mode 100644
index 0000000..f880e19
--- /dev/null
+++ b/crafts.lua
@@ -0,0 +1,64 @@
+--[[
+
+  Copyright (C) 2013 PilzAdam
+  Copyright (C) 2020 lortas
+  Copyright (C) 2020 Treer
+
+  Permission to use, copy, modify, and/or distribute this software for
+  any purpose with or without fee is hereby granted, provided that the
+  above copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+  WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+  WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR
+  BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
+  OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+  WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+  ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
+  SOFTWARE.
+
+]]--
+
+minetest.register_craft({
+	output = "nether:brick 4",
+	recipe = {
+		{"nether:rack", "nether:rack"},
+		{"nether:rack", "nether:rack"},
+	}
+})
+
+minetest.register_craft({
+	output = "nether:fence_nether_brick 6",
+	recipe = {
+		{"nether:brick", "nether:brick", "nether:brick"},
+		{"nether:brick", "nether:brick", "nether:brick"},
+	},
+})
+
+minetest.register_craft({
+	output = "nether:brick_compressed",
+	recipe = {
+		{"nether:brick","nether:brick","nether:brick"},
+		{"nether:brick","nether:brick","nether:brick"},
+		{"nether:brick","nether:brick","nether:brick"},
+	}
+})
+
+minetest.register_craft({
+    output = "nether:basalt_hewn",
+    type = "shapeless",
+    recipe = {
+      "nether:basalt",
+      "nether:basalt",
+    },
+})
+
+minetest.register_craft({
+  output = "nether:basalt_chiselled 4",
+  recipe = {
+    {"nether:basalt_hewn", "nether:basalt_hewn"},
+    {"nether:basalt_hewn", "nether:basalt_hewn"}
+  }
+})
+
+-- See tools.lua for tools related crafting
\ No newline at end of file
diff --git a/depends.txt b/depends.txt
index a6ab237..00f426a 100644
--- a/depends.txt
+++ b/depends.txt
@@ -1,8 +1,8 @@
-stairs
 default
-moreblocks?
-mesecons?
-loot?
-dungeon_loot?
+stairs
 doc_basics?
+dungeon_loot?
 fire?
+loot?
+mesecons?
+moreblocks?
diff --git a/init.lua b/init.lua
index c6e24c1..c09234e 100644
--- a/init.lua
+++ b/init.lua
@@ -116,6 +116,7 @@ end
 dofile(nether.path .. "/portal_api.lua")
 dofile(nether.path .. "/nodes.lua")
 dofile(nether.path .. "/tools.lua")
+dofile(nether.path .. "/crafts.lua")
 if nether.NETHER_REALM_ENABLED then
 	if nether.useBiomes then
 		dofile(nether.path .. "/mapgen.lua")
@@ -227,3 +228,18 @@ The expedition parties have found no diamonds or gold, and after an experienced
 
 	})
 end
+
+
+-- Play bubbling lava sounds if player killed by lava
+minetest.register_on_dieplayer(
+	function(player, reason)
+		if reason.node ~= nil and minetest.get_node_group(reason.node, "lava") > 0 or reason.node == "nether:lava_crust" then
+			minetest.sound_play(
+				"nether_lava_bubble",
+				-- this sample was encoded at 3x speed to reduce .ogg file size
+				-- at the expense of higher frequencies, so pitch it down ~3x
+				{to_player = player:get_player_name(), pitch = 0.3, gain = 0.8}
+			)
+		end
+	end
+)
\ No newline at end of file
diff --git a/mapgen.lua b/mapgen.lua
index b449c5c..abc145a 100644
--- a/mapgen.lua
+++ b/mapgen.lua
@@ -2,6 +2,12 @@
 
   Nether mod for minetest
 
+  "mapgen.lua" is the modern biomes-based Nether mapgen, which
+    requires Minetest v5.1 or greater
+  "mapgen_nobiomes.lua" is the legacy version of the mapgen, only used
+    in older versions of Minetest or in v6 worlds.
+
+
   Copyright (C) 2013 PilzAdam
 
   Permission to use, copy, modify, and/or distribute this software for
@@ -27,10 +33,32 @@ local NETHER_FLOOR   = nether.DEPTH_FLOOR
 local TCAVE = 0.6
 local BLEND = 128
 
+-- parameters for central region
+local REGION_BUFFER_THICKNESS   = 0.2
+local CENTER_REGION_LIMIT       = TCAVE - REGION_BUFFER_THICKNESS -- Netherrack gives way to Deep-Netherrack here
+local CENTER_CAVERN_LIMIT       = CENTER_REGION_LIMIT - 0.1       -- Deep-Netherrack gives way to air here
+local SURFACE_CRUST_LIMIT       = CENTER_CAVERN_LIMIT * 0.25      -- Crusted-lava at the surface of the lava ocean gives way to liquid lava here
+local CRUST_LIMIT               = CENTER_CAVERN_LIMIT * 0.85      -- Crusted-lava under the surface of the lava ocean gives way to liquid lava here
+local BASALT_COLUMN_UPPER_LIMIT = CENTER_CAVERN_LIMIT * 0.9       -- Basalt columns may appear between these upper and lower limits
+local BASALT_COLUMN_LOWER_LIMIT = CENTER_CAVERN_LIMIT * 0.25      -- This value is close to SURFACE_CRUST_LIMIT so basalt columns give way to "flowing" lava rivers
 
--- Stuff
 
-local math_max, math_min = math.max, math.min -- avoid needing table lookups each time a common math function is invoked
+-- Shared Nether mapgen namespace
+-- For mapgen files to share functions and constants
+nether.mapgen = {}
+local mapgen = nether.mapgen
+
+mapgen.TCAVE                     = TCAVE                     -- const needed in mapgen_mantle.lua
+mapgen.BLEND                     = BLEND                     -- const needed in mapgen_mantle.lua
+mapgen.CENTER_REGION_LIMIT       = CENTER_REGION_LIMIT       -- const needed in mapgen_mantle.lua
+mapgen.CENTER_CAVERN_LIMIT       = CENTER_CAVERN_LIMIT       -- const needed in mapgen_mantle.lua
+mapgen.BASALT_COLUMN_UPPER_LIMIT = BASALT_COLUMN_UPPER_LIMIT -- const needed in mapgen_mantle.lua
+mapgen.BASALT_COLUMN_LOWER_LIMIT = BASALT_COLUMN_LOWER_LIMIT -- const needed in mapgen_mantle.lua
+
+mapgen.ore_ceiling = NETHER_CEILING - BLEND -- leave a solid 128 node cap of netherrack before introducing ores
+mapgen.ore_floor   = NETHER_FLOOR   + BLEND
+
+local debugf = nether.debug
 
 if minetest.read_schematic == nil then
 	-- Using biomes to create the Nether requires the ability for biomes to set "node_cave_liquid = air".
@@ -41,6 +69,18 @@ if minetest.read_schematic == nil then
 	error("This " .. nether.modname .. " mapgen requires Minetest v5.1 or greater, use mapgen_nobiomes.lua instead.", 0)
 end
 
+-- Load helper functions for generating the mantle / center region
+dofile(nether.path .. "/mapgen_mantle.lua")
+
+
+-- Misc math functions
+
+-- avoid needing table lookups each time a common math function is invoked
+local math_max, math_min, math_abs, math_floor = math.max, math.min, math.abs, math.floor
+
+
+-- Inject nether_caverns biome
+
 local function override_underground_biomes()
 	-- https://forum.minetest.net/viewtopic.php?p=257522#p257522
 	-- Q: Is there a way to override an already-registered biome so I can get it out of the
@@ -123,7 +163,7 @@ minetest.register_biome({
 	node_stone  = "nether:native_mapgen", -- nether:native_mapgen is used here to prevent the native mapgen from placing ores and decorations.
 	node_filler = "nether:native_mapgen", -- The lua on_generate will transform nether:rack_native into nether:rack then decorate and add ores.
 	node_dungeon = "nether:brick",
-	--node_dungeon_alt = "default:mossycobble",
+	node_dungeon_alt = "nether:brick_cracked",
 	node_dungeon_stair = "stairs:stair_nether_brick",
 	-- Setting node_cave_liquid to "air" avoids the need to filter lava and water out of the mapchunk and
 	-- surrounding shell (overdraw nodes beyond the mapchunk).
@@ -152,19 +192,30 @@ minetest.register_ore({
 	clust_scarcity = 11 * 11 * 11,
 	clust_num_ores = 3,
 	clust_size     = 2,
-	y_max = NETHER_CEILING,
-	y_min = NETHER_FLOOR,
+	y_max = mapgen.ore_ceiling,
+	y_min = mapgen.ore_floor
+})
+
+minetest.register_ore({
+	ore_type       = "scatter",
+	ore            = "nether:lava_crust", -- crusted lava replaces of scattered glowstone in the mantle
+	wherein        = "nether:rack_deep",
+	clust_scarcity = 16 * 16 * 16,
+	clust_num_ores = 4,
+	clust_size     = 2,
+	y_max = mapgen.ore_ceiling,
+	y_min = mapgen.ore_floor
 })
 
 minetest.register_ore({
 	ore_type       = "scatter",
 	ore            = "default:lava_source",
-	wherein        = "nether:rack",
+	wherein        = {"nether:rack", "nether:rack_deep"},
 	clust_scarcity = 36 * 36 * 36,
 	clust_num_ores = 4,
 	clust_size     = 2,
-	y_max = NETHER_CEILING,
-	y_min = NETHER_FLOOR,
+	y_max = mapgen.ore_ceiling,
+	y_min = mapgen.ore_floor
 })
 
 minetest.register_ore({
@@ -173,8 +224,8 @@ minetest.register_ore({
 	wherein         = "nether:rack",
 	clust_scarcity  = 14 * 14 * 14,
 	clust_size      = 8,
-	y_max = NETHER_CEILING,
-	y_min = NETHER_FLOOR
+	y_max = mapgen.ore_ceiling,
+	y_min = mapgen.ore_floor
 })
 
 
@@ -182,7 +233,7 @@ minetest.register_ore({
 
 -- 3D noise
 
-local np_cave = {
+mapgen.np_cave = {
 	offset = 0,
 	scale = 1,
 	spread = {x = 384, y = 128, z = 384}, -- squashed 3:1
@@ -199,23 +250,29 @@ local nobj_cave = nil
 local nbuf_cave = {}
 local dbuf = {}
 
-local yblmin = NETHER_FLOOR   + BLEND * 2
-local yblmax = NETHER_CEILING - BLEND * 2
 
 -- Content ids
 
 local c_air              = minetest.get_content_id("air")
 local c_netherrack       = minetest.get_content_id("nether:rack")
-local c_netherbrick      = minetest.get_content_id("nether:brick")
+local c_netherrack_deep  = minetest.get_content_id("nether:rack_deep")
+local c_dungeonbrick     = minetest.get_content_id("nether:brick")
+local c_dungeonbrick_alt = minetest.get_content_id("nether:brick_cracked")
 local c_netherbrick_slab = minetest.get_content_id("stairs:slab_nether_brick")
 local c_netherfence      = minetest.get_content_id("nether:fence_nether_brick")
 local c_glowstone        = minetest.get_content_id("nether:glowstone")
 local c_lava_source      = minetest.get_content_id("default:lava_source")
+local c_lavasea_source   = minetest.get_content_id("nether:lava_source") -- same as lava but with staggered animation to look better as an ocean
+local c_lava_crust       = minetest.get_content_id("nether:lava_crust")
 local c_native_mapgen    = minetest.get_content_id("nether:native_mapgen")
 
 
 -- Dungeon excavation functions
 
+function is_dungeon_brick(node_id)
+	return node_id == c_dungeonbrick or node_id == c_dungeonbrick_alt
+end
+
 function build_dungeon_room_list(data, area)
 
 	local result = {}
@@ -310,6 +367,8 @@ end
 -- (Corridors and the parts of rooms which extend beyond the emerge boundary will remain filled)
 function excavate_dungeons(data, area, rooms)
 
+	local vi, node_id
+
 	-- any air from the native mapgen has been replaced by netherrack, but
 	-- we don't want this inside dungeons, so fill dungeon rooms with air
 	for _, roomInfo in ipairs(rooms) do
@@ -319,9 +378,10 @@ function excavate_dungeons(data, area, rooms)
 
 		for z = room_min.z, room_max.z do
 			for y = room_min.y, room_max.y do
-				local vi = area:index(room_min.x, y, z)
+				vi = area:index(room_min.x, y, z)
 				for x = room_min.x, room_max.x do
-					if data[vi] == c_netherrack then data[vi] = c_air end
+					node_id = data[vi]
+					if node_id == c_netherrack or node_id == c_netherrack_deep then data[vi] = c_air end
 					vi = vi + 1
 				end
 			end
@@ -346,20 +406,22 @@ function decorate_dungeons(data, area, rooms)
 			local window_y  = roomInfo.y + math_min(2, room_max.y - roomInfo.y - 1)
 
 			if room_seed % 3 == 0 and room_max.y < maxEdge.y then
-				-- Glowstone chandelier
+				-- Glowstone chandelier (feel free to replace with a fancy schematic)
 				local vi = area:index(roomInfo.x, room_max.y + 1, roomInfo.z)
-				if data[vi] == c_netherbrick then data[vi] = c_glowstone end
+				if is_dungeon_brick(data[vi]) then data[vi] = c_glowstone end
 
 			elseif room_seed % 4 == 0 and room_min.y > minEdge.y
 				   and room_min.x > minEdge.x and room_max.x < maxEdge.x
 				   and room_min.z > minEdge.z and room_max.z < maxEdge.z then
 				-- lava well (feel free to replace with a fancy schematic)
 				local vi = area:index(roomInfo.x, room_min.y, roomInfo.z)
-				if data[vi - yStride] == c_netherbrick then data[vi - yStride] = c_lava_source end
-				if data[vi - zStride] == c_air then data[vi - zStride] = c_netherbrick_slab end
-				if data[vi + zStride] == c_air then data[vi + zStride] = c_netherbrick_slab end
-				if data[vi - xStride] == c_air then data[vi - xStride] = c_netherbrick_slab end
-				if data[vi + xStride] == c_air then data[vi + xStride] = c_netherbrick_slab end
+				if is_dungeon_brick(data[vi - yStride]) then
+					data[vi - yStride] = c_lava_source
+					if data[vi - zStride] == c_air then data[vi - zStride] = c_netherbrick_slab end
+					if data[vi + zStride] == c_air then data[vi + zStride] = c_netherbrick_slab end
+					if data[vi - xStride] == c_air then data[vi - xStride] = c_netherbrick_slab end
+					if data[vi + xStride] == c_air then data[vi + xStride] = c_netherbrick_slab end
+				end
 			end
 
 			-- Barred windows
@@ -377,15 +439,15 @@ function decorate_dungeons(data, area, rooms)
 				local vi_max = area:index(room_max.x + 1, window_y, roomInfo.z)
 				local locations = {-zStride, zStride, -zStride + yStride, zStride + yStride}
 				for _, offset in ipairs(locations) do
-					if data[vi_min + offset] == c_netherbrick then data[vi_min + offset] = window_node end
-					if data[vi_max + offset] == c_netherbrick then data[vi_max + offset] = window_node end
+					if is_dungeon_brick(data[vi_min + offset]) then data[vi_min + offset] = window_node end
+					if is_dungeon_brick(data[vi_max + offset]) then data[vi_max + offset] = window_node end
 				end
 				vi_min = area:index(roomInfo.x, window_y, room_min.z - 1)
 				vi_max = area:index(roomInfo.x, window_y, room_max.z + 1)
 				locations = {-xStride, xStride, -xStride + yStride, xStride + yStride}
 				for _, offset in ipairs(locations) do
-					if data[vi_min + offset] == c_netherbrick then data[vi_min + offset] = window_node end
-					if data[vi_max + offset] == c_netherbrick then data[vi_max + offset] = window_node end
+					if is_dungeon_brick(data[vi_min + offset]) then data[vi_min + offset] = window_node end
+					if is_dungeon_brick(data[vi_max + offset]) then data[vi_max + offset] = window_node end
 				end
 			end
 
@@ -395,8 +457,43 @@ function decorate_dungeons(data, area, rooms)
 end
 
 
+local yblmin = NETHER_FLOOR   + BLEND * 2
+local yblmax = NETHER_CEILING - BLEND * 2
+-- At both the top and bottom of the Nether, as set by NETHER_CEILING and NETHER_FLOOR,
+-- there is a 128 deep cap of solid netherrack, followed by a 128-deep blending zone
+-- where Nether caverns may start to appear.
+-- The solid zones and blending zones are achieved by adjusting the np_cave noise to be
+-- outside the range where caverns form, this function returns that adjustment.
+--
+-- Returns two values: the noise limit adjustment for nether caverns, and the
+-- noise limit adjustment for the central region / mantle caverns
+mapgen.get_mapgenblend_adjustments = function(y)
+
+	-- floorAndCeilingBlend will normally be 0, but shifts toward 1 in the
+	-- blending zone, and goes higher than 1 in the solid zone between the
+	-- blending zone and the end of the nether.
+	local floorAndCeilingBlend = 0
+	if y > yblmax then floorAndCeilingBlend = ((y - yblmax) / BLEND) ^ 2 end
+	if y < yblmin then floorAndCeilingBlend = ((yblmin - y) / BLEND) ^ 2 end
+
+	-- the nether caverns exist when np_cave noise is greater than TCAVE, so
+	-- to fade out the nether caverns, adjust TCAVE upward.
+	local tcave_adj               = floorAndCeilingBlend
+
+	-- the central regions exists when np_cave noise is below CENTER_REGION_LIMIT,
+	-- so to fade out the mantle caverns adjust CENTER_REGION_LIMIT downward.
+	local centerRegionLimit_adj = -(CENTER_REGION_LIMIT * floorAndCeilingBlend)
+
+	return tcave_adj, centerRegionLimit_adj
+end
+
+
+
 -- On-generated function
 
+local tunnelCandidate_count = 0
+local tunnel_count = 0
+local total_chunk_count = 0
 local function on_generated(minp, maxp, seed)
 
 	if minp.y > NETHER_CEILING or maxp.y < NETHER_FLOOR then
@@ -414,38 +511,120 @@ local function on_generated(minp, maxp, seed)
 	local zCaveStride = yCaveStride * yCaveStride
 	local chulens = {x = yCaveStride, y = yCaveStride, z = yCaveStride}
 
-	nobj_cave = nobj_cave or minetest.get_perlin_map(np_cave, chulens)
+	nobj_cave = nobj_cave or minetest.get_perlin_map(mapgen.np_cave, chulens)
 	local nvals_cave = nobj_cave:get_3d_map_flat(minp, nbuf_cave)
 
-
 	local dungeonRooms = build_dungeon_room_list(data, area)
+	local abs_cave_noise, abs_cave_noise_adjusted
 
-	for y = y0, y1 do -- Y loop first to minimise tcave calculations
+	local contains_nether = false
+	local contains_shell  = false
+	local contains_mantle = false
+	local contains_ocean = false
 
-		local tcave = TCAVE
-		if y > yblmax then tcave = TCAVE + ((y - yblmax) / BLEND) ^ 2 end
-		if y < yblmin then tcave = TCAVE + ((yblmin - y) / BLEND) ^ 2 end
+
+	for y = y0, y1 do -- Y loop first to minimise tcave & lava-sea calculations
+
+		local sea_level, cavern_limit_distance = mapgen.find_nearest_lava_sealevel(y)
+		local above_lavasea = y > sea_level
+		local below_lavasea = y < sea_level
+
+		local tcave_adj, centerRegionLimit_adj = mapgen.get_mapgenblend_adjustments(y)
+		local tcave   = TCAVE + tcave_adj
+		local tmantle = CENTER_REGION_LIMIT + centerRegionLimit_adj -- cavern_noise_adj already contains central_region_limit_adj, so tmantle is only for comparisons when cavern_noise_adj hasn't been added to the noise value
+		local cavern_noise_adj =
+			CENTER_REGION_LIMIT * (cavern_limit_distance * cavern_limit_distance * cavern_limit_distance) -
+			centerRegionLimit_adj -- cavern_noise_adj gets added to noise value instead of added to the limit np_noise is compared against, so subtract centerRegionLimit_adj so subtract centerRegionLimit_adj instead of adding
 
 		for z = z0, z1 do
 			local vi = area:index(x0, y, z) -- Initial voxelmanip index
 			local ni = (z - z0) * zCaveStride + (y - y0) * yCaveStride + 1
+			local noise2di = 1 + (z - z0) * yCaveStride
 
 			for x = x0, x1 do
 
-				local id = data[vi] -- Existing node
+				local cave_noise = nvals_cave[ni]
 
-				if nvals_cave[ni] > tcave then
+				if cave_noise > tcave then
+					-- Prime region
+					-- This was the only region in initial versions of the Nether mod.
+					-- It is the only region that portals from the surface will open into.
 					data[vi] = c_air
-				elseif id == c_air or id == c_native_mapgen then
-					data[vi] = c_netherrack -- excavate_dungeons() will mostly reverse this inside dungeons
+					contains_nether = true
+
+				elseif -cave_noise > tcave then
+					-- Secondary/spare region
+					-- This secondary region is unused until someone decides to do something cool or novel with it.
+					-- Reaching here would require the player to first find and journey through the central region,
+					-- as it's always separated from the Prime region by the central region.
+
+					data[vi] = c_netherrack -- For now I've just left this region as solid netherrack instead of air.
+
+					-- Only set contains_nether to true here if you want tunnels created between the secondary region
+					-- and the central region.
+					--contains_nether = true
+					--data[vi] = c_air
+				else
+					-- netherrack walls and/or center region/mantle
+					abs_cave_noise = math_abs(cave_noise)
+
+					-- abs_cave_noise_adjusted makes the center region smaller as distance from the lava ocean
+					-- increases, we do this by pretending the abs_cave_noise value is higher.
+					abs_cave_noise_adjusted = abs_cave_noise + cavern_noise_adj
+
+					if abs_cave_noise_adjusted >= CENTER_CAVERN_LIMIT then
+
+						local id = data[vi] -- Check existing node to avoid removing dungeons
+						if id == c_air or id == c_native_mapgen then
+							if abs_cave_noise < tmantle then
+								data[vi] = c_netherrack_deep
+							else
+								-- the shell seperating the mantle from the rest of the nether...
+								data[vi] = c_netherrack -- excavate_dungeons() will mostly reverse this inside dungeons
+								contains_shell = true
+							end
+						end
+
+					elseif above_lavasea then
+						data[vi] = c_air
+						contains_mantle = true
+					elseif abs_cave_noise_adjusted < SURFACE_CRUST_LIMIT or (below_lavasea and abs_cave_noise_adjusted < CRUST_LIMIT) then
+						data[vi] = c_lavasea_source
+						contains_ocean = true
+					else
+						data[vi] = c_lava_crust
+						contains_ocean = true
+					end
 				end
 
 				ni = ni + 1
 				vi = vi + 1
+				noise2di = noise2di + 1
 			end
 		end
 	end
 
+	if contains_mantle or contains_ocean then
+		mapgen.add_basalt_columns(data, area, minp, maxp) -- function provided by mapgen_mantle.lua
+	end
+
+	if contains_nether and contains_mantle then
+		tunnelCandidate_count = tunnelCandidate_count + 1
+		local success = mapgen.excavate_tunnel_to_center_of_the_nether(data, area, nvals_cave, minp, maxp) -- function provided by mapgen_mantle.lua
+		if success then tunnel_count = tunnel_count + 1 end
+	end
+	total_chunk_count = total_chunk_count + 1
+	if total_chunk_count % 50 == 0 then
+		debugf(
+			"%s of %s chunks contain both nether and lava-sea (%s%%), %s chunks generated a pathway (%s%%)",
+			tunnelCandidate_count,
+			total_chunk_count,
+			math_floor(tunnelCandidate_count * 100 / total_chunk_count),
+			tunnel_count,
+			math_floor(tunnel_count * 100 / total_chunk_count)
+		)
+	end
+
 	-- any air from the native mapgen has been replaced by netherrack, but we
 	-- don't want netherrack inside dungeons, so fill known dungeon rooms with air.
 	excavate_dungeons(data, area, dungeonRooms)
@@ -453,10 +632,9 @@ local function on_generated(minp, maxp, seed)
 
 	vm:set_data(data)
 
-	-- avoid generating decorations on the underside of the bottom of the nether
-	if minp.y > NETHER_FLOOR and maxp.y < NETHER_CEILING then minetest.generate_decorations(vm) end
-
 	minetest.generate_ores(vm)
+	minetest.generate_decorations(vm)
+
 	vm:set_lighting({day = 0, night = 0}, minp, maxp)
 	vm:calc_lighting()
 	vm:update_liquids()
@@ -467,7 +645,7 @@ end
 -- use knowledge of the nether mapgen algorithm to return a suitable ground level for placing a portal.
 -- player_name is optional, allowing a player to spawn a remote portal in their own protected areas.
 function nether.find_nether_ground_y(target_x, target_z, start_y, player_name)
-	local nobj_cave_point = minetest.get_perlin(np_cave)
+	local nobj_cave_point = minetest.get_perlin(mapgen.np_cave)
 	local air = 0 -- Consecutive air nodes found
 
 	local minp_schem, maxp_schem = nether.get_schematic_volume({x = target_x, y = 0, z = target_z}, nil, "nether_portal")
diff --git a/mapgen_decorations.lua b/mapgen_decorations.lua
index 7005da4..5ceb5a6 100644
--- a/mapgen_decorations.lua
+++ b/mapgen_decorations.lua
@@ -25,10 +25,16 @@
 -- emerged or not before the decoration was placed.
 local allow_lava_decorations = nether.useBiomes
 
+-- Keep compatibility with mapgen_nobiomes.lua, so hardcoding 128
+-- instead of using nether.mapgen.BLEND
+local decoration_ceiling = nether.DEPTH_CEILING - 128
+local decoration_floor   = nether.DEPTH_FLOOR   + 128
+
 local _  = {name = "air",                     prob = 0}
 local A  = {name = "air",                     prob = 255, force_place = true}
 local G  = {name = "nether:glowstone",        prob = 255, force_place = true}
 local N  = {name = "nether:rack",             prob = 255}
+local D  = {name = "nether:rack_deep",        prob = 255}
 local S  = {name = "nether:sand",             prob = 255, force_place = true}
 local L  = {name = "default:lava_source",     prob = 255, force_place = true}
 local F  = {name = "nether:fumarole",         prob = 255, force_place = true}
@@ -125,8 +131,8 @@ minetest.register_decoration({
     sidelen = 80,
     fill_ratio = 0.0003,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = schematic_GlowstoneStalactite,
     flags = "place_center_x,place_center_z,force_placement,all_ceilings",
     place_offset_y=-3
@@ -139,8 +145,8 @@ minetest.register_decoration({
     sidelen = 80,
     fill_ratio = 0.0008,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = schematic_GlowstoneStalactite,
     replacements = {["nether:glowstone"] = "nether:rack"},
     flags = "place_center_x,place_center_z,all_ceilings",
@@ -148,6 +154,193 @@ minetest.register_decoration({
 })
 
 
+local schematic_GreaterStalactite = {
+    size = {x = 3, y = 23, z = 3},
+    data = { -- note that data is upside down
+
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, D, _,
+        _, D, _,
+        _, D, _,
+        _, D, _,
+        D, D, D,
+        D, D, D,
+        D, D, D,
+        _, D, _,
+        _, _, _,
+        _, _, _,
+
+        _, D, _,  -- ypos 0, prob 85% (218/255)
+        _, D, _,  -- ypos 1, prob 85% (218/255)
+        _, D, _,  -- ypos 2, prob 85% (218/255)
+        _, D, _,  -- ypos 3, prob 85% (218/255)
+        _, D, _,  -- ypos 4, prob 85% (218/255)
+        _, D, _,  -- ypos 5, prob 85% (218/255)
+        _, D, _,  -- ypos 6, prob 85% (218/255)
+        _, D, _,  -- ypos 7, prob 85% (218/255)
+        _, D, _,  -- ypos 8, prob 85% (218/255)
+        _, D, D,  -- ypos 9, prob 50% (128/256) to make half of stalactites asymmetric
+        _, D, D,  -- ypos 10, prob 50% (128/256) to make half of stalactites asymmetric
+        _, D, D,  -- ypos 11, prob 50% (128/256) to make half of stalactites asymmetric
+        _, D, D,  -- ypos 12, prob 50% (128/256) to make half of stalactites asymmetric
+        D, D, D,  -- ypos 13, prob 75% (192/256)
+        D, D, D,  -- ypos 14, prob 75% (192/256)
+        D, D, D,  -- ypos 15, prob 100%
+        D, D, D,  -- ypos 16, prob 100%
+        D, D, D,  -- ypos 17, prob 100%
+        D, D, D,  -- ypos 18, prob 100%
+        D, D, D,  -- ypos 19, prob 75% (192/256)
+        D, D, D,  -- ypos 20, prob 85% (218/255)
+        _, D, D,  -- ypos 21, prob 50% (128/256) to make half of stalactites asymmetric
+        _, D, _,  -- ypos 22, prob 100%
+
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, _, _,
+        _, D, _,
+        _, D, _,
+        _, D, _,
+        _, D, _,
+        _, D, _,
+        D, D, D,
+        D, D, D,
+        D, D, D,
+        _, D, _,
+        _, D, _,
+        _, _, _,
+
+    },
+    -- Y-slice probabilities do not function correctly for ceiling schematic
+    -- decorations because they are inverted, so ypos numbers have been inverted
+    -- to match, and a larger offset in place_offset_y should be used (e.g. -3).
+    yslice_prob = {
+        {ypos = 21, prob = 128},
+        {ypos = 20, prob = 218},
+        {ypos = 19, prob = 192},
+        {ypos = 14, prob = 192},
+        {ypos = 13, prob = 192},
+        {ypos = 12, prob = 128},
+        {ypos = 11, prob = 128},
+        {ypos = 10, prob = 128},
+        {ypos =  9, prob = 128},
+        {ypos =  8, prob = 218},
+        {ypos =  7, prob = 218},
+        {ypos =  6, prob = 218},
+        {ypos =  5, prob = 218},
+        {ypos =  4, prob = 218},
+        {ypos =  3, prob = 218},
+        {ypos =  2, prob = 218},
+        {ypos =  1, prob = 218},
+        {ypos =  0, prob = 218}
+    }
+}
+
+
+
+-- A stalagmite is an upsidedown stalactite, so
+-- use the GreaterStalactite to create a ToweringStalagmite schematic
+local schematic_ToweringStalagmite = {
+    size = schematic_GreaterStalactite.size,
+    data = {},
+    yslice_prob = {}
+}
+local array_length = #schematic_GreaterStalactite.data + 1
+for i, node in ipairs(schematic_GreaterStalactite.data) do
+    schematic_ToweringStalagmite.data[array_length - i] = node
+end
+y_size = schematic_GreaterStalactite.size.y
+for i, node in ipairs(schematic_GreaterStalactite.yslice_prob) do
+    schematic_ToweringStalagmite.yslice_prob[i] = {
+        -- we can safely lower the prob. to gain more variance because floor based schematics
+        -- don't have the bug where missing lines moves them away from the surface
+        prob = schematic_GreaterStalactite.yslice_prob[i].prob - 20,
+        ypos = y_size - 1 - schematic_GreaterStalactite.yslice_prob[i].ypos
+    }
+end
+
+minetest.register_decoration({
+    name = "Deep-glowstone stalactite",
+    deco_type = "schematic",
+    place_on = "nether:rack_deep",
+    sidelen = 80,
+    fill_ratio = 0.0003,
+    biomes = {"nether_caverns"},
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
+    schematic = schematic_GlowstoneStalactite,
+    replacements = {["nether:rack"] = "nether:rack_deep", ["nether:glowstone"] = "nether:glowstone_deep"},
+    flags = "place_center_x,place_center_z,force_placement,all_ceilings",
+    place_offset_y=-3
+})
+
+minetest.register_decoration({
+    name = "Deep-glowstone stalactite outgrowth",
+    deco_type = "schematic",
+    place_on = "nether:glowstone_deep",
+    sidelen = 40,
+    fill_ratio = 0.15,
+    biomes = {"nether_caverns"},
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
+    schematic = {
+        size = {x = 1, y = 4, z = 1},
+        data = { G, G, G, G }
+    },
+    replacements = {["nether:glowstone"] = "nether:glowstone_deep"},
+    flags = "place_center_x,place_center_z,all_ceilings",
+})
+
+minetest.register_decoration({
+    name = "Deep-netherrack stalactite",
+    deco_type = "schematic",
+    place_on = "nether:rack_deep",
+    sidelen = 80,
+    fill_ratio = 0.0003,
+    biomes = {"nether_caverns"},
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
+    schematic = schematic_GlowstoneStalactite,
+    replacements = {["nether:rack"] = "nether:rack_deep", ["nether:glowstone"] = "nether:rack_deep"},
+    flags = "place_center_x,place_center_z,force_placement,all_ceilings",
+    place_offset_y=-3
+})
+
+minetest.register_decoration({
+    name = "Deep-netherrack towering stalagmite",
+    deco_type = "schematic",
+    place_on = "nether:rack_deep",
+    sidelen = 80,
+    fill_ratio = 0.001,
+    biomes = {"nether_caverns"},
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
+    schematic = schematic_ToweringStalagmite,
+    replacements = {["nether:basalt"] = "nether:rack_deep"},
+    flags = "place_center_x,place_center_z,force_placement,all_floors",
+    place_offset_y=-2
+})
+
 -- =======================================
 --    Concealed crevice / Lava sinkhole
 -- =======================================
@@ -161,8 +354,8 @@ if allow_lava_decorations then
         sidelen = 80,
         fill_ratio = 0.002,
         biomes = {"nether_caverns"},
-        y_max = nether.DEPTH, -- keep compatibility with mapgen_nobiomes.lua
-        y_min = nether.DEPTH_FLOOR,
+        y_max = decoration_ceiling,
+        y_min = decoration_floor,
         schematic = {
             size = {x = 4, y = 7, z = 4},
             data = { -- note that data is upside down
@@ -263,8 +456,8 @@ minetest.register_decoration({
     sidelen = 80,
     fill_ratio = 0.005,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH,
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = schematic_fumarole,
     replacements = replacements_full,
     flags = "place_center_x,place_center_z,all_floors",
@@ -292,8 +485,8 @@ minetest.register_decoration({
     sidelen = 8,
     noise_params = fumarole_clump_noise,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH,
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = schematic_fumarole,
     replacements = replacements_full,
     flags = "place_center_x,place_center_z,all_floors",
@@ -308,8 +501,8 @@ minetest.register_decoration({
     sidelen = 8,
     noise_params = fumarole_clump_noise,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH,
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = schematic_fumarole,
     replacements = replacements_slab,
     flags = "place_center_x,place_center_z,all_floors",
@@ -324,8 +517,8 @@ minetest.register_decoration({
     sidelen = 8,
     noise_params = fumarole_clump_noise,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH,
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = {
         size = {x = 4, y = 4, z = 4},
         data = { -- note that data is upside down
@@ -363,8 +556,8 @@ minetest.register_decoration({
     sidelen = 8,
     noise_params = fumarole_clump_noise,
     biomes = {"nether_caverns"},
-    y_max = nether.DEPTH,
-    y_min = nether.DEPTH_FLOOR,
+    y_max = decoration_ceiling,
+    y_min = decoration_floor,
     schematic = {
         size = {x = 4, y = 5, z = 4},
         data = { -- note that data is upside down
diff --git a/mapgen_mantle.lua b/mapgen_mantle.lua
new file mode 100644
index 0000000..0add888
--- /dev/null
+++ b/mapgen_mantle.lua
@@ -0,0 +1,476 @@
+--[[
+
+  Nether mod for minetest
+
+  This file contains helper functions for generating the Mantle
+  (AKA center region), which are moved into a separate file to keep the
+  size of mapgen.lua manageable.
+
+
+  Copyright (C) 2021 Treer
+
+  Permission to use, copy, modify, and/or distribute this software for
+  any purpose with or without fee is hereby granted, provided that the
+  above copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+  WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+  WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR
+  BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
+  OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+  WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+  ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
+  SOFTWARE.
+
+]]--
+
+
+local debugf = nether.debug
+local mapgen = nether.mapgen
+
+local BASALT_COLUMN_UPPER_LIMIT = mapgen.BASALT_COLUMN_UPPER_LIMIT
+local BASALT_COLUMN_LOWER_LIMIT = mapgen.BASALT_COLUMN_LOWER_LIMIT
+
+
+-- 2D noise for basalt formations
+local np_basalt = {
+	offset      =-0.85,
+	scale       = 1,
+	spread      = {x = 46, y = 46, z = 46},
+	seed        = 1000,
+	octaves     = 5,
+	persistence = 0.5,
+	lacunarity  = 2.6,
+	flags = "eased"
+}
+
+
+-- Buffers and objects we shouldn't recreate every on_generate
+
+local nobj_basalt = nil
+local nbuf_basalt = {}
+local cavePerlin = nil
+
+-- Content ids
+
+local c_air              = minetest.get_content_id("air")
+local c_netherrack_deep  = minetest.get_content_id("nether:rack_deep")
+local c_glowstone        = minetest.get_content_id("nether:glowstone")
+local c_lavasea_source   = minetest.get_content_id("nether:lava_source") -- same as lava but with staggered animation to look better as an ocean
+local c_lava_crust       = minetest.get_content_id("nether:lava_crust")
+local c_basalt           = minetest.get_content_id("nether:basalt")
+
+
+-- Math funcs
+local math_max, math_min, math_abs, math_floor = math.max, math.min, math.abs, math.floor -- avoid needing table lookups each time a common math function is invoked
+
+function random_unit_vector()
+	return vector.normalize({
+		x = math.random() - 0.5,
+		y = math.random() - 0.5,
+		z = math.random() - 0.5
+	})
+end
+
+-- returns the smallest component in the vector
+function vector_min(v)
+	return math_min(v.x, math_min(v.y, v.z))
+end
+
+
+-- Mantle mapgen functions (AKA Center region)
+
+-- Returns (absolute height, fractional distance from ceiling or sea floor)
+-- the fractional distance from ceiling or sea floor is a value between 0 and 1 (inclusive)
+-- Note it may find the most relevent sea-level - not necesssarily the one you are closest
+-- to, since the space above the sea reaches much higher than the depth below the sea.
+mapgen.find_nearest_lava_sealevel = function(y)
+	-- todo: put oceans near the bottom of chunks to improve ability to generate tunnels to the center
+	-- todo: constrain y to be not near the bounds of the nether
+	-- todo: add some random adj at each level, seeded only by the level height
+	local sealevel = math.floor((y + 100) / 200) * 200
+	--local sealevel = math.floor((y + 80) / 160) * 160
+	--local sealevel = math.floor((y + 120) / 240) * 240
+
+	local cavern_limits_fraction
+	local height_above_sea = y - sealevel
+	if height_above_sea >= 0 then
+		cavern_limits_fraction = math_min(1, height_above_sea / 95)
+	else
+		-- approaches 1 much faster as the lava sea is shallower than the cavern above it
+		cavern_limits_fraction = math_min(1, -height_above_sea / 40)
+	end
+
+	return sealevel, cavern_limits_fraction
+end
+
+
+
+
+mapgen.add_basalt_columns = function(data, area, minp, maxp)
+	-- Basalt columns are structures found in lava oceans, and the only way to obtain
+	-- nether basalt.
+	-- Their x, z position is determined by a 2d noise map and a 2d slice of the cave
+	-- noise (taken at lava-sealevel).
+
+	local x0, y0, z0 = minp.x, math_max(minp.y, nether.DEPTH_FLOOR),   minp.z
+	local x1, y1, z1 = maxp.x, math_min(maxp.y, nether.DEPTH_CEILING), maxp.z
+
+	local yStride = area.ystride
+	local yCaveStride = x1 - x0 + 1
+
+	cavePerlin = cavePerlin or minetest.get_perlin(mapgen.np_cave)
+	nobj_basalt = nobj_basalt or minetest.get_perlin_map(np_basalt, {x = yCaveStride, y = yCaveStride})
+	local nvals_basalt = nobj_basalt:get_2d_map_flat({x=minp.x, y=minp.z}, {x=yCaveStride, y=yCaveStride}, nbuf_basalt)
+
+	local nearest_sea_level, _ = mapgen.find_nearest_lava_sealevel(math_floor((y0 + y1) / 2))
+
+	local leeway = mapgen.CENTER_CAVERN_LIMIT * 0.18
+
+	for z = z0, z1 do
+		local noise2di = 1 + (z - z0) * yCaveStride
+
+		for x = x0, x1 do
+
+			local basaltNoise = nvals_basalt[noise2di]
+			if basaltNoise > 0 then
+				-- a basalt column is here
+
+				local abs_sealevel_cave_noise = math_abs(cavePerlin:get3d({x = x, y = nearest_sea_level, z = z}))
+
+				-- Add Some quick deterministic noise to the column heights
+				-- This is probably not good noise, but it doesn't have to be.
+				local fastNoise = 17
+				fastNoise = 37 * fastNoise + y0
+				fastNoise = 37 * fastNoise + z
+				fastNoise = 37 * fastNoise + x
+				fastNoise = 37 * fastNoise + math_floor(basaltNoise * 32)
+
+				local columnHeight = basaltNoise * 18 + ((fastNoise % 3) - 1)
+
+				-- columns should drop below sealevel where lava rivers are flowing
+				-- i.e. anywhere abs_sealevel_cave_noise < BASALT_COLUMN_LOWER_LIMIT
+				-- And we'll also have it drop off near the edges of the lava ocean so that
+				-- basalt columns can only be found by the player reaching a lava ocean.
+				local lowerClip = (math_min(math_max(abs_sealevel_cave_noise, BASALT_COLUMN_LOWER_LIMIT - leeway), BASALT_COLUMN_LOWER_LIMIT + leeway) - BASALT_COLUMN_LOWER_LIMIT) / leeway
+				local upperClip = (math_min(math_max(abs_sealevel_cave_noise, BASALT_COLUMN_UPPER_LIMIT - leeway), BASALT_COLUMN_UPPER_LIMIT + leeway) - BASALT_COLUMN_UPPER_LIMIT) / leeway
+				local columnHeightAdj = lowerClip * -upperClip -- all are values between 1 and -1
+
+				columnHeight = columnHeight + math_floor(columnHeightAdj * 12 - 12)
+
+				local vi = area:index(x, y0, z) -- Initial voxelmanip index
+
+				for y = y0, y1 do -- Y loop first to minimise tcave & lava-sea calculations
+
+					if y < nearest_sea_level + columnHeight  then
+
+						local id = data[vi] -- Existing node
+						if id == c_lava_crust or id == c_lavasea_source or (id == c_air and y > nearest_sea_level) then
+							-- Avoid letting columns extend beyond the central region.
+							-- (checking node ids saves having to calculate abs_cave_noise_adjusted here
+							-- to test it against CENTER_CAVERN_LIMIT)
+							data[vi] = c_basalt
+						end
+					end
+
+					vi = vi + yStride
+				end
+			end
+
+			noise2di = noise2di + 1
+		end
+	end
+end
+
+
+-- returns an array of points from pos1 and pos2 which deviate from a straight line
+-- but which don't venture too close to a chunk boundary
+function generate_waypoints(pos1, pos2, minp, maxp)
+
+	local segSize = 10
+	local maxDeviation = 7
+	local minDistanceFromChunkWall = 5
+
+	local pathVec     = vector.subtract(pos2, pos1)
+	local pathVecNorm = vector.normalize(pathVec)
+	local pathLength  = vector.distance(pos1, pos2)
+	local minBound    = vector.add(minp, minDistanceFromChunkWall)
+	local maxBound    = vector.subtract(maxp, minDistanceFromChunkWall)
+
+	local result = {}
+	result[1] = pos1
+
+	local segmentCount = math_floor(pathLength / segSize)
+	for i = 1, segmentCount do
+		local waypoint = vector.add(pos1, vector.multiply(pathVec, i / (segmentCount + 1)))
+
+		-- shift waypoint a few blocks in a random direction orthogonally to the pathVec, to make the path crooked.
+		local crossProduct
+		repeat
+			crossProduct = vector.normalize(vector.cross(pathVecNorm, random_unit_vector()))
+		until vector.length(crossProduct) > 0
+		local deviation = vector.multiply(crossProduct, math.random(1, maxDeviation))
+		waypoint = vector.add(waypoint, deviation)
+		waypoint = {
+			x = math_min(maxBound.x, math_max(minBound.x, waypoint.x)),
+			y = math_min(maxBound.y, math_max(minBound.y, waypoint.y)),
+			z = math_min(maxBound.z, math_max(minBound.z, waypoint.z))
+		}
+
+		result[#result + 1] = waypoint
+	end
+
+	result[#result + 1] = pos2
+	return result
+end
+
+
+function excavate_pathway(data, area, nether_pos, center_pos, minp, maxp)
+
+	local ystride = area.ystride
+	local zstride = area.zstride
+
+	math.randomseed(nether_pos.x + 10 * nether_pos.y + 100 * nether_pos.z) -- so each tunnel generates deterministically (this doesn't have to be a quality seed)
+	local dist = math_floor(vector.distance(nether_pos, center_pos))
+	local waypoints = generate_waypoints(nether_pos, center_pos, minp, maxp)
+
+	-- First pass: record path details
+	local linedata = {}
+	local last_pos = {}
+	local line_index = 1
+	local first_filled_index, boundary_index, last_filled_index
+	for i = 0, dist do
+		-- Bresenham's line would be good here, but too much lua code
+		local waypointProgress = (#waypoints - 1) * i / dist
+		local segmentIndex = math_min(math_floor(waypointProgress) + 1, #waypoints - 1) -- from the integer portion of waypointProgress
+		local segmentInterp = waypointProgress - (segmentIndex - 1)                     -- the remaining fractional portion
+		local segmentStart = waypoints[segmentIndex]
+		local segmentVector = vector.subtract(waypoints[segmentIndex + 1], segmentStart)
+		local pos = vector.round(vector.add(segmentStart, vector.multiply(segmentVector, segmentInterp)))
+
+		if not vector.equals(pos, last_pos) then
+			local vi = area:indexp(pos)
+			local node_id = data[vi]
+			linedata[line_index] = {
+				pos = pos,
+				vi = vi,
+				node_id = node_id
+			}
+			if boundary_index == nil and node_id == c_netherrack_deep then
+				boundary_index = line_index
+			end
+			if node_id == c_air then
+				if boundary_index ~= nil and last_filled_index == nil then
+					last_filled_index = line_index
+				end
+			else
+				if first_filled_index == nil then
+					first_filled_index = line_index
+				end
+			end
+			line_index = line_index + 1
+			last_pos = pos
+		end
+	end
+	first_filled_index = first_filled_index or 1
+	last_filled_index  = last_filled_index  or #linedata
+	boundary_index     = boundary_index     or last_filled_index
+
+
+	-- limit tunnel radius to roughly the closest that startPos or stopPos comes to minp-maxp, so we
+	-- don't end up exceeding minp-maxp and having excavation filled in when the next chunk is generated.
+	local startPos, stopPos = linedata[first_filled_index].pos, linedata[last_filled_index].pos
+	local radiusLimit = vector_min(vector.subtract(startPos, minp))
+	radiusLimit = math_min(radiusLimit, vector_min(vector.subtract(stopPos, minp)))
+	radiusLimit = math_min(radiusLimit, vector_min(vector.subtract(maxp, startPos)))
+	radiusLimit = math_min(radiusLimit, vector_min(vector.subtract(maxp, stopPos)))
+
+	if radiusLimit < 4 then -- This is a logic check, ignore it. It could be commented out
+		-- 4 is (79 - 75), and shouldn't be possible if sampling-skip was 10
+		-- i.e. if sampling-skip was 10 then {5, 15, 25, 35, 45, 55, 65, 75} should be sampled from possible positions 0 to 79
+		debugf("Error: radiusLimit %s is smaller then half the sampling distance. min %s, max %s, start %s, stop %s", radiusLimit, minp, maxp, startPos, stopPos)
+	end
+	radiusLimit = radiusLimit + 1 -- chunk walls wont be visibly flat if the radius only exceeds it a little ;)
+
+	-- Second pass: excavate
+	local start_index, stop_index = math_max(1, first_filled_index - 2), math_min(#linedata, last_filled_index + 3)
+	for i = start_index, stop_index, 3 do
+
+		-- Adjust radius so that tunnels start wide but thin out in the middle
+		local distFromEnds = 1 - math_abs(((start_index + stop_index) / 2) - i) / ((stop_index - start_index) / 2) -- from 0 to 1, with 0 at ends and 1 in the middle
+		-- Have it more flaired at the ends, rather than linear.
+		-- i.e. sizeAdj approaches 1 quickly as distFromEnds increases
+		local distFromMiddle = 1 - distFromEnds
+		local sizeAdj = 1 - (distFromMiddle * distFromMiddle * distFromMiddle)
+
+		local radius = math_min(radiusLimit, math.random(50 - (25 * sizeAdj), 80 - (45 * sizeAdj)) / 10)
+		local radiusCubed = radius * radius
+		local radiusCeil = math_floor(radius + 0.5)
+
+		linedata[i].radius = radius             -- Needed in third pass
+		linedata[i].distFromEnds = distFromEnds -- Needed in third pass
+
+		local vi = linedata[i].vi
+		for z = -radiusCeil, radiusCeil do
+			local vi_z = vi + z * zstride
+			for y = -radiusCeil, radiusCeil do
+				local vi_zy = vi_z + y * ystride
+				local xSquaredLimit = radiusCubed - (z * z + y * y)
+				for x = -radiusCeil, radiusCeil do
+					if x * x < xSquaredLimit then
+						data[vi_zy + x] = c_air
+					end
+				end
+			end
+		end
+
+	end
+
+	-- Third pass: decorate
+	-- Add glowstones to make tunnels to the mantle easyier to find
+	-- https://i.imgur.com/sRA28x7.jpg
+	for i = start_index, stop_index, 3 do
+		if linedata[i].distFromEnds < 0.3 then
+			local glowcount = 0
+			local radius = linedata[i].radius
+			for _ = 1, 20 do
+				local testPos = vector.round(vector.add(linedata[i].pos, vector.multiply(random_unit_vector(), radius + 0.5)))
+				local vi = area:indexp(testPos)
+				if data[vi] ~= c_air then
+					data[vi] = c_glowstone
+					glowcount = glowcount + 1
+				--else
+				--	data[vi] = c_debug
+				end
+				if glowcount >= 2 then break end
+			end
+		end
+	end
+
+end
+
+
+-- excavates a tunnel connecting the Primary or Secondary region with the mantle / central region
+-- if a suitable path is found.
+-- Returns true if successful
+mapgen.excavate_tunnel_to_center_of_the_nether = function(data, area, nvals_cave, minp, maxp)
+
+	local result = false
+	local extent = vector.subtract(maxp, minp)
+	local skip = 10 -- sampling rate of 1 in 10
+
+	local highest = -1000
+	local lowest  =  1000
+	local lowest_vi
+	local highest_vi
+
+	local yCaveStride = maxp.x - minp.x + 1
+	local zCaveStride = yCaveStride * yCaveStride
+
+	local vi_offset = area:indexp(vector.add(minp, math_floor(skip / 2))) -- start half the sampling distance away from minp
+	local vi, ni
+
+	for y = 0, extent.y - 1, skip do
+		local sealevel = mapgen.find_nearest_lava_sealevel(minp.y + y)
+
+		if minp.y + y > sealevel then -- only create tunnels above sea level
+			for z = 0, extent.z - 1, skip do
+
+				vi = vi_offset + y * area.ystride + z * area.zstride
+				ni = z * zCaveStride + y * yCaveStride + 1
+				for x = 0, extent.x - 1, skip do
+
+					local noise = math_abs(nvals_cave[ni])
+					if noise < lowest then
+						lowest = noise
+						lowest_vi = vi
+					end
+					if noise > highest then
+						highest = noise
+						highest_vi = vi
+					end
+					ni = ni + skip
+					vi = vi + skip
+				end
+			end
+		end
+	end
+
+	if lowest < mapgen.CENTER_CAVERN_LIMIT and highest > mapgen.TCAVE + 0.03 then
+
+		local mantle_y = area:position(lowest_vi).y
+		local sealevel, cavern_limit_distance = mapgen.find_nearest_lava_sealevel(mantle_y)
+		local _, centerRegionLimit_adj = mapgen.get_mapgenblend_adjustments(mantle_y)
+		local cavern_noise_adj =
+			mapgen.CENTER_REGION_LIMIT * (cavern_limit_distance * cavern_limit_distance * cavern_limit_distance) -
+			centerRegionLimit_adj -- cavern_noise_adj gets added to noise value instead of added to the limit np_noise is compared against, so subtract centerRegionLimit_adj instead of adding
+
+		if lowest + cavern_noise_adj < mapgen.CENTER_CAVERN_LIMIT then
+			excavate_pathway(data, area, area:position(highest_vi), area:position(lowest_vi), minp, maxp)
+			result = true
+		end
+	end
+	return result
+end
+
+
+minetest.register_chatcommand("nether_whereami",
+    {
+		description = "Describes which region of the nether the player is in",
+		privs = {debug = true},
+		func = function(name, param)
+
+			local player = minetest.get_player_by_name(name)
+			if player == nil then return false, "Unknown player position" end
+
+			local pos = vector.round(player:get_pos())
+			if pos.y > nether.DEPTH_CEILING or pos.y < nether.DEPTH_FLOOR then
+				return true, "The Overworld"
+			end
+
+			cavePerlin = cavePerlin or minetest.get_perlin(mapgen.np_cave)
+			local densityNoise = cavePerlin:get_3d(pos)
+			local sea_level, cavern_limit_distance = mapgen.find_nearest_lava_sealevel(pos.y)
+			local tcave_adj, centerRegionLimit_adj = mapgen.get_mapgenblend_adjustments(pos.y)
+			local tcave   = mapgen.TCAVE + tcave_adj
+			local tmantle = mapgen.CENTER_REGION_LIMIT + centerRegionLimit_adj
+			local cavern_noise_adj =
+				mapgen.CENTER_REGION_LIMIT * (cavern_limit_distance * cavern_limit_distance * cavern_limit_distance) -
+				centerRegionLimit_adj -- cavern_noise_adj gets added to noise value instead of added to the limit np_noise is compared against, so subtract centerRegionLimit_adj so subtract centerRegionLimit_adj instead of adding
+
+			local desc
+
+			if densityNoise > tcave then
+				desc = "Positive nether"
+			elseif -densityNoise > tcave then
+				desc = "Negative nether"
+			elseif math_abs(densityNoise) < tmantle then
+				desc =  "Mantle"
+
+				if math_abs(densityNoise) + cavern_noise_adj < mapgen.CENTER_CAVERN_LIMIT then
+					desc =  desc .. " inside cavern"
+				else
+					desc =  desc .. " but outside cavern"
+				end
+
+			elseif densityNoise > 0 then
+				desc =  "Shell between positive nether and center region"
+			else
+				desc = "Shell between negative nether and center region"
+			end
+
+			local sea_pos = pos.y - sea_level
+			if sea_pos > 0 then
+				desc = desc .. ", " .. sea_pos .. "m above lava-sea level"
+			else
+				desc = desc .. ", " .. sea_pos .. "m below lava-sea level"
+			end
+
+			if tcave_adj > 0 then
+				desc = desc .. ", approaching y boundary of Nether"
+			end
+
+			return true, "[Perlin " .. (math_floor(densityNoise * 1000) / 1000) .. "] " .. desc
+		end
+	}
+)
diff --git a/mapgen_nobiomes.lua b/mapgen_nobiomes.lua
index 103537d..28792b8 100644
--- a/mapgen_nobiomes.lua
+++ b/mapgen_nobiomes.lua
@@ -2,6 +2,12 @@
 
   Nether mod for minetest
 
+  "mapgen_nobiomes.lua" is the legacy version of the mapgen, only used
+    in older versions of Minetest or in v6 worlds.
+  "mapgen.lua" is the modern biomes-based Nether mapgen, which
+    requires Minetest v5.1 or greater
+
+
   Copyright (C) 2013 PilzAdam
 
   Permission to use, copy, modify, and/or distribute this software for
@@ -190,8 +196,7 @@ minetest.register_on_generated(function(minp, maxp, seed)
 
 	vm:set_data(data)
 
-	-- avoid generating decorations on the underside of the bottom of the nether
-	if minp.y > NETHER_FLOOR and maxp.y < NETHER_CEILING then minetest.generate_decorations(vm) end
+	minetest.generate_decorations(vm)
 
 	vm:set_lighting({day = 0, night = 0}, minp, maxp)
 	vm:calc_lighting()
diff --git a/nodes.lua b/nodes.lua
index ea94206..2af360e 100644
--- a/nodes.lua
+++ b/nodes.lua
@@ -75,6 +75,15 @@ minetest.register_node("nether:rack", {
 	sounds = default.node_sound_stone_defaults(),
 })
 
+-- Deep Netherrack, found in the mantle / central magma layers
+minetest.register_node("nether:rack_deep", {
+	description = S("Deep-Netherrack"),
+	tiles = {"nether_rack_deep.png"},
+	is_ground_content = true,
+	groups = {cracky = 3, level = 2},
+	sounds = default.node_sound_stone_defaults(),
+})
+
 minetest.register_node("nether:sand", {
 	description = S("Nethersand"),
 	tiles = {"nether_sand.png"},
@@ -95,6 +104,17 @@ minetest.register_node("nether:glowstone", {
 	sounds = default.node_sound_glass_defaults(),
 })
 
+-- Deep glowstone, found in the mantle / central magma layers
+minetest.register_node("nether:glowstone_deep", {
+	description = S("Deep-Glowstone"),
+	tiles = {"nether_glowstone_deep.png"},
+	is_ground_content = true,
+	light_source = 14,
+	paramtype = "light",
+	groups = {cracky = 3, oddly_breakable_by_hand = 3},
+	sounds = default.node_sound_glass_defaults(),
+})
+
 minetest.register_node("nether:brick", {
 	description = S("Nether Brick"),
 	tiles = {"nether_brick.png"},
@@ -111,6 +131,15 @@ minetest.register_node("nether:brick_compressed", {
 	sounds = default.node_sound_stone_defaults(),
 })
 
+-- A decorative node which can only be obtained from dungeons or structures
+minetest.register_node("nether:brick_cracked", {
+	description = S("Cracked Nether Brick"),
+	tiles = {"nether_brick_cracked.png"},
+	is_ground_content = false,
+	groups = {cracky = 2, level = 2},
+	sounds = default.node_sound_stone_defaults(),
+})
+
 local fence_texture =
 	"default_fence_overlay.png^nether_brick.png^default_fence_overlay.png^[makealpha:255,126,126"
 
@@ -169,6 +198,345 @@ if minetest.get_modpath("moreblocks") then
 end
 
 
+-- Mantle nodes
+
+-- Nether basalt is intended as a valuable material and possible portalstone - an alternative to
+-- obsidian that's available for other mods to use.
+-- It cannot be found in the regions of the nether where Nether portals link to, so requires a journey to obtain.
+minetest.register_node("nether:basalt", {
+	description = S("Blue Basalt"),
+	tiles = {
+		"nether_basalt.png",
+		"nether_basalt.png",
+		"nether_basalt_side.png",
+		"nether_basalt_side.png",
+		"nether_basalt_side.png",
+		"nether_basalt_side.png"
+	},
+	is_ground_content = true,
+	groups = {cracky = 1, level = 3}, -- set proper digging times and uses, and maybe explosion immune if api handles that
+	on_blast = function() --[[blast proof]] end,
+	sounds = default.node_sound_stone_defaults(),
+})
+
+-- Potentially a portalstone, but will also be a stepping stone between basalt
+-- and chiseled basalt.
+-- It can only be introduced by the biomes-based mapgen, since it requires the
+-- MT 5.0 world-align texture features.
+minetest.register_node("nether:basalt_hewn", {
+	description = S("Hewn Basalt"),
+	tiles = {{
+		name        = "nether_basalt_hewn.png",
+		align_style = "world",
+		scale       = 2
+	}},
+	inventory_image = minetest.inventorycube(
+		"nether_basalt_hewn.png^[sheet:2x2:0,0",
+		"nether_basalt_hewn.png^[sheet:2x2:0,1",
+		"nether_basalt_hewn.png^[sheet:2x2:1,1"
+	),
+	is_ground_content = false,
+	groups = {cracky = 1, level = 2},
+	on_blast = function() --[[blast proof]] end,
+	sounds = default.node_sound_stone_defaults(),
+})
+
+-- Chiselled basalt is intended as a portalstone - an alternative to obsidian that's
+-- available for other mods to use. It is crafted from Hewn Basalt.
+-- It should only be introduced by the biomes-based mapgen, since in future it may
+-- require the MT 5.0 world-align texture features.
+minetest.register_node("nether:basalt_chiselled", {
+	description = S("Chiselled Basalt"),
+	tiles = {
+		"nether_basalt_chiselled_top.png",
+		"nether_basalt_chiselled_top.png" .. "^[transformFY",
+		"nether_basalt_chiselled_side.png",
+		"nether_basalt_chiselled_side.png",
+		"nether_basalt_chiselled_side.png",
+		"nether_basalt_chiselled_side.png"
+	},
+	inventory_image = minetest.inventorycube(
+		"nether_basalt_chiselled_top.png",
+		"nether_basalt_chiselled_side.png",
+		"nether_basalt_chiselled_side.png"
+	),
+	paramtype2 = "facedir",
+	is_ground_content = false,
+	groups = {cracky = 1, level = 2},
+	on_blast = function() --[[blast proof]] end,
+	sounds = default.node_sound_stone_defaults(),
+})
+
+
+-- Lava-sea source
+-- This is a lava source using a different animated texture so that each node
+-- is out of phase in its animation from its neighbor. This prevents the magma
+-- ocean from visually clumping together into a patchwork of 16x16 squares.
+-- It can only be used by the biomes-based mapgen, since it requires the MT 5.0
+-- world-align texture features.
+local lavasea_source = {}
+local lava_source = minetest.registered_nodes["default:lava_source"]
+for key, value in pairs(lava_source) do lavasea_source[key] = value end
+lavasea_source.name = nil
+lavasea_source.tiles = {
+	{
+		name             = "nether_lava_source_animated.png",
+		backface_culling = false,
+		align_style      = "world",
+		scale            = 2,
+		animation = {
+			type     = "vertical_frames",
+			aspect_w = 32,
+			aspect_h = 32,
+			length   = 3.0,
+		},
+	},
+	{
+		name             = "nether_lava_source_animated.png",
+		backface_culling = true,
+		align_style      = "world",
+		scale            = 2,
+		animation = {
+			type     = "vertical_frames",
+			aspect_w = 32,
+			aspect_h = 32,
+			length   = 3.0,
+		},
+	},
+}
+lavasea_source.liquid_alternative_source = "nether:lava_source"
+lavasea_source.inventory_image = minetest.inventorycube(
+	"nether_lava_source_animated.png^[sheet:2x16:0,0",
+	"nether_lava_source_animated.png^[sheet:2x16:0,1",
+	"nether_lava_source_animated.png^[sheet:2x16:1,1"
+)
+minetest.register_node("nether:lava_source", lavasea_source)
+
+
+-- a place to store the original ABM function so nether.cool_lava() can call it
+local original_cool_lava_action
+
+nether.cool_lava = function(pos, node)
+
+	local pos_above = {x = pos.x, y = pos.y + 1, z = pos.z}
+	local node_above = minetest.get_node(pos_above)
+
+	-- Evaporate water sitting above lava, if it's in the Nether.
+	-- (we don't want Nether mod to affect overworld lava mechanics)
+	if minetest.get_node_group(node_above.name, "water") > 0 and
+		pos.y < nether.DEPTH_CEILING and pos.y > nether.DEPTH_FLOOR then
+		-- cools_lava might be a better group to check for, but perhaps there's
+		-- something in that group that isn't a liquid and shouldn't be evaporated?
+		minetest.swap_node(pos_above, {name="air"})
+	end
+
+	-- add steam to cooling lava
+	minetest.add_particlespawner({
+		amount = 20,
+		time = 0.15,
+		minpos = {x=pos.x - 0.4, y=pos.y - 0,   z=pos.z - 0.4},
+		maxpos = {x=pos.x + 0.4, y=pos.y + 0.5, z=pos.z + 0.4},
+		minvel = {x = -0.5, y = 0.5, z = -0.5},
+		maxvel = {x =  0.5, y = 1.5, z =  0.5},
+		minacc = {x = 0, y = 0.1, z = 0},
+		maxacc = {x = 0, y = 0.2, z = 0},
+		minexptime = 0.5,
+		maxexptime = 1.3,
+		minsize = 1.5,
+		maxsize = 3.5,
+		texture = "nether_particle_anim4.png",
+		animation = {
+			type = "vertical_frames",
+			aspect_w = 7,
+			aspect_h = 7,
+			length = 1.4,
+		}
+	})
+
+	if node.name == "nether:lava_source" or node.name == "nether:lava_crust" then
+		-- use swap_node to avoid triggering the lava_crust's after_destruct
+		minetest.swap_node(pos, {name = "nether:basalt"})
+
+		minetest.sound_play("default_cool_lava",
+			{pos = pos, max_hear_distance = 16, gain = 0.25}, true)
+	else
+		-- chain the original ABM action to handle conventional lava
+		original_cool_lava_action(pos, node)
+	end
+end
+
+
+minetest.register_on_mods_loaded(function()
+
+	-- register a bucket of Lava-sea source - but make it just the same bucket as default lava.
+	-- (by doing this in register_on_mods_loaded we don't need to declare a soft dependency)
+	if minetest.get_modpath("bucket") and minetest.global_exists("bucket") then
+		local lava_bucket = bucket.liquids["default:lava_source"]
+		if lava_bucket ~= nil then
+			local lavasea_bucket = {}
+			for key, value in pairs(lava_bucket) do lavasea_bucket[key] = value end
+			lavasea_bucket.source = "nether:lava_source"
+			bucket.liquids[lavasea_bucket.source] = lavasea_bucket
+		end
+	end
+
+	-- include "nether:lava_source" in any "default:lava_source" ABMs
+	local function include_nether_lava(set_of_nodes)
+		if (type(set_of_nodes) == "table") then
+			for _, nodename in pairs(set_of_nodes) do
+				if nodename == "default:lava_source" then
+					-- I'm amazed this works, but it does
+					table.insert(set_of_nodes, "nether:lava_source")
+					break;
+				end
+			end
+		end
+	end
+
+	for _, abm in pairs(minetest.registered_abms) do
+		include_nether_lava(abm.nodenames)
+		include_nether_lava(abm.neighbors)
+		if abm.label == "Lava cooling" and abm.action ~= nil then
+			-- lets have lava_crust cool as well
+			original_cool_lava_action = abm.action
+			abm.action = nether.cool_lava
+			table.insert(abm.nodenames, "nether:lava_crust")
+		end
+	end
+	for _, lbm in pairs(minetest.registered_lbms) do
+		include_nether_lava(lbm.nodenames)
+	end
+	--minetest.log("minetest.registered_abms" .. dump(minetest.registered_abms))
+	--minetest.log("minetest.registered_lbms" .. dump(minetest.registered_lbms))
+end)
+
+-- creates a lava splash, and leaves lava_source in place of the lava_crust
+local function smash_lava_crust(pos, playsound)
+
+	local lava_particlespawn_def = {
+		amount = 6,
+		time = 0.1,
+		minpos = {x=pos.x - 0.5, y=pos.y + 0.3, z=pos.z - 0.5},
+		maxpos = {x=pos.x + 0.5, y=pos.y + 0.5, z=pos.z + 0.5},
+		minvel = {x = -1.5, y = 1.5, z = -1.5},
+		maxvel = {x =  1.5, y = 5,   z =  1.5},
+		minacc = {x = 0, y = -10, z = 0},
+		maxacc = {x = 0, y = -10, z = 0},
+		minexptime = 1,
+		maxexptime = 1,
+		minsize = .2,
+		maxsize = .8,
+		texture = "^[colorize:#A00:255",
+		glow = 8
+	}
+	minetest.add_particlespawner(lava_particlespawn_def)
+	lava_particlespawn_def.texture = "^[colorize:#FB0:255"
+	lava_particlespawn_def.maxvel.y = 3
+	lava_particlespawn_def.glow = 12
+	minetest.add_particlespawner(lava_particlespawn_def)
+
+	minetest.set_node(pos, {name = "default:lava_source"})
+
+	if math.random(1, 3) == 1 and minetest.registered_nodes["fire:basic_flame"] ~= nil then
+		-- occasionally brief flames will be seen when breaking lava crust
+		local posAbove = {x = pos.x, y = pos.y + 1, z = pos.z}
+		if minetest.get_node(posAbove).name == "air" then
+			minetest.set_node(posAbove, {name = "fire:basic_flame"})
+			minetest.get_node_timer(posAbove):set(math.random(7, 15) / 10, 0)
+			--[[ commented out because the flame sound plays for too long
+			if minetest.global_exists("fire") and fire.update_player_sound ~= nil then
+				-- The fire mod only updates its sound every 3 seconds, these flames will be
+				-- out by then, so start the sound immediately
+				local players = minetest.get_connected_players()
+				for n = 1, #players do fire.update_player_sound(players[n]) end
+			end]]
+		end
+	end
+
+	if playsound then
+		minetest.sound_play(
+			"nether_lava_bubble",
+			-- this sample was encoded at 3x speed to reduce .ogg file size
+			-- at the expense of higher frequencies, so pitch it down ~3x
+			{pos = pos, pitch = 0.3, max_hear_distance = 8, gain = 0.4}
+		)
+	end
+end
+
+
+-- lava_crust nodes can only be used in the biomes-based mapgen, since they require
+-- the MT 5.0 world-align texture features.
+minetest.register_node("nether:lava_crust", {
+	description = "Lava crust",
+	tiles = {
+		{
+			name="nether_lava_crust_animated.png",
+			backface_culling=true,
+			tileable_vertical=true,
+			tileable_horizontal=true,
+			align_style="world",
+			scale=2,
+			animation = {
+				type = "vertical_frames",
+				aspect_w = 32,
+				aspect_h = 32,
+				length = 1.8,
+			},
+		}
+	},
+	inventory_image = minetest.inventorycube(
+		"nether_lava_crust_animated.png^[sheet:2x48:0,0",
+		"nether_lava_crust_animated.png^[sheet:2x48:0,1",
+		"nether_lava_crust_animated.png^[sheet:2x48:1,1"
+	),
+	collision_box = {
+		type = "fixed",
+		fixed = {
+			-- Damage is calculated "starting 0.1 above feet
+			-- and progressing upwards in 1 node intervals", so
+			-- lower this node's collision box by more than 0.1
+			-- to ensure damage will be taken when standing on
+			-- the node.
+			{-0.5, -0.5, -0.5, 0.5, 0.39, 0.5}
+		},
+	},
+	selection_box = {
+		type = "fixed",
+		fixed = {
+			-- Keep the selection box matching the visual node,
+			-- rather than the collision_box.
+			{-0.5, -0.5, -0.5, 0.5, 0.5, 0.5}
+		},
+	},
+
+	after_destruct = function(pos, oldnode)
+		smash_lava_crust(pos, true)
+	end,
+	after_dig_node = function(pos, oldnode, oldmetadata, digger)
+	end,
+	on_blast = function(pos, intensity)
+		smash_lava_crust(pos, false)
+	end,
+
+	paramtype = "light",
+	light_source = default.LIGHT_MAX - 3,
+	buildable_to = false,
+	walkable_to = true,
+	is_ground_content = true,
+	drop = {
+		items = {{
+			-- Allow SilkTouch-esque "pickaxes of preservation" to mine the lava crust intact, if PR #10141 gets merged.
+			tools = {"this line will block early MT versions which don't respect the tool_groups restrictions"},
+			tool_groups = {{"pickaxe", "preservation"}},
+			items = {"nether:lava_crust"}
+		}}
+	},
+	--liquid_viscosity = 7,
+	damage_per_second = 2,
+	groups = {oddly_breakable_by_hand = 3, cracky = 3, explody = 1, igniter = 1},
+})
+
+
 -- Fumaroles (Chimney's)
 
 local function fumarole_startTimer(pos, timeout_factor)
@@ -393,31 +761,3 @@ local airlike_darkness = {}
 for k,v in pairs(minetest.registered_nodes["air"]) do airlike_darkness[k] = v end
 airlike_darkness.paramtype = "none"
 minetest.register_node("nether:airlike_darkness", airlike_darkness)
-
-
--- Crafting
-
-minetest.register_craft({
-	output = "nether:brick 4",
-	recipe = {
-		{"nether:rack", "nether:rack"},
-		{"nether:rack", "nether:rack"},
-	}
-})
-
-minetest.register_craft({
-	output = "nether:fence_nether_brick 6",
-	recipe = {
-		{"nether:brick", "nether:brick", "nether:brick"},
-		{"nether:brick", "nether:brick", "nether:brick"},
-	},
-})
-
-minetest.register_craft({
-	output = "nether:brick_compressed",
-	recipe = {
-		{"nether:brick","nether:brick","nether:brick"},
-		{"nether:brick","nether:brick","nether:brick"},
-		{"nether:brick","nether:brick","nether:brick"},
-	}
-})
diff --git a/sounds/nether_lava_bubble.0.ogg b/sounds/nether_lava_bubble.0.ogg
new file mode 100644
index 0000000..4eeef69
Binary files /dev/null and b/sounds/nether_lava_bubble.0.ogg differ
diff --git a/sounds/nether_lava_bubble.ogg b/sounds/nether_lava_bubble.ogg
new file mode 100644
index 0000000..44a5031
Binary files /dev/null and b/sounds/nether_lava_bubble.ogg differ
diff --git a/textures/nether_basalt.png b/textures/nether_basalt.png
new file mode 100644
index 0000000..08a348f
Binary files /dev/null and b/textures/nether_basalt.png differ
diff --git a/textures/nether_basalt_chiselled_side.png b/textures/nether_basalt_chiselled_side.png
new file mode 100644
index 0000000..46a173a
Binary files /dev/null and b/textures/nether_basalt_chiselled_side.png differ
diff --git a/textures/nether_basalt_chiselled_top.png b/textures/nether_basalt_chiselled_top.png
new file mode 100644
index 0000000..3d6a70e
Binary files /dev/null and b/textures/nether_basalt_chiselled_top.png differ
diff --git a/textures/nether_basalt_hewn.png b/textures/nether_basalt_hewn.png
new file mode 100644
index 0000000..60db5e3
Binary files /dev/null and b/textures/nether_basalt_hewn.png differ
diff --git a/textures/nether_basalt_side.png b/textures/nether_basalt_side.png
new file mode 100644
index 0000000..7c33847
Binary files /dev/null and b/textures/nether_basalt_side.png differ
diff --git a/textures/nether_brick_cracked.png b/textures/nether_brick_cracked.png
new file mode 100644
index 0000000..1729b2c
Binary files /dev/null and b/textures/nether_brick_cracked.png differ
diff --git a/textures/nether_glowstone_deep.png b/textures/nether_glowstone_deep.png
new file mode 100644
index 0000000..0e37b8c
Binary files /dev/null and b/textures/nether_glowstone_deep.png differ
diff --git a/textures/nether_lava_crust_animated.png b/textures/nether_lava_crust_animated.png
new file mode 100644
index 0000000..f8f8c25
Binary files /dev/null and b/textures/nether_lava_crust_animated.png differ
diff --git a/textures/nether_lava_source_animated.png b/textures/nether_lava_source_animated.png
new file mode 100644
index 0000000..c8514ad
Binary files /dev/null and b/textures/nether_lava_source_animated.png differ
diff --git a/textures/nether_particle_anim4.png b/textures/nether_particle_anim4.png
new file mode 100644
index 0000000..0f1b849
Binary files /dev/null and b/textures/nether_particle_anim4.png differ
diff --git a/textures/nether_rack_deep.png b/textures/nether_rack_deep.png
new file mode 100644
index 0000000..69bb7c2
Binary files /dev/null and b/textures/nether_rack_deep.png differ
diff --git a/tools.lua b/tools.lua
index 637cdf0..26e1425 100644
--- a/tools.lua
+++ b/tools.lua
@@ -1,3 +1,22 @@
+--[[
+
+  Copyright (C) 2020 lortas
+
+  Permission to use, copy, modify, and/or distribute this software for
+  any purpose with or without fee is hereby granted, provided that the
+  above copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+  WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+  WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR
+  BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
+  OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+  WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+  ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
+  SOFTWARE.
+
+]]--
+
 local S = nether.get_translator
 
 minetest.register_tool("nether:pick_nether", {