This is a modified version of lkjoel's nether mod.
Look here if you want to see the differences:
https://github.com/HybridDog/minetest-nether/compare/lkjoel:master...master

this happens really selden to me
http://i.imgur.com/pMZYqt9.png


TODO:
— find a way to get the perlin noise inside [-1; 1] or use another noise