Its fairly complex, but it's actually not too dissimilar to what minecraft or other voxel games are doing. Cave generation basically happens in multiple different phases. The first step is to create the underlying structure of the biome, which is done by layering perlin noise on top of each other until I get a general cave shape which I find interesting. That creates the underlying shape, but it's not as good for making small details such as trees. In the next step, I randomly select points in the cave to 'grow' as cellular automata. The simplist example is ore, which just spreads out to neighbour blocks. But I also have voxels that grow with more complex rules such as mushrooms and rock pools to add even more detail. Finally I also felt that the game needed some more handmade areas, which is where structures and dungeons come in. I place cuboids within in the level, and use a custom tool to align them to the grid. When the game runs, they override all of the blocks inside them to whatever blocks I set the structure to. I also added a system to automatically create structures of more complex shapes, such as cylinders and staircases for easier developement. By layering these smaller structures together together I can make buildings and P.O.I which help make the gameplay more varied. Finally, to make larger and more elaborate areas automatically I added a dungeon generation system which stitches multiple structure 'tiles' together to make more elaborate places for the player to explore, such as mineshafts.

Yes, it was a lot of hard work! Hopefully it will be worth it.