I'm creating a fantasy-world generator. So far I've got it generating a reasonably good surface map, complete with terrain, cities, and borders. Here's a link for anybody who wants to try it: https://drive.google.com/open?id=0Bxgb9V1OuYfpUFdmOWRxb2VrOGM. This is a super early build, but still feedback is appreciated! The basic program workflow is:
Generate height map (mountain ranges, coastlines)
Generate Terrain from Height Map. For the surface, this involves placing forests, wilderness, fields, swamps, etc, in places that would make sense given the mountains/oceans.
Generate provinces, cities, and kingdoms based around the generated geography.
I have this working pretty well for the surface-world, but one thing I want to do is expand the map downwards to include an Underdark-like world below. The methodology I have for making the height-map is all good, but I'm at a bit of a loss over what types of terrain/biomes to use to fill in this underworld. In terms of geography, it will contain a maze of wide-open caverns, with large underground lakes. There will be points here and there where this connects with the surface-world and travel/trade is possible. To give you an impression of the scale I'm talking about, the underworld would line up exactly with the map generated in the example above, the underground lakes would be roughly the same size as the surface ones, and the caverns would be 3-8 “blocks” wide (1 block = 1 small coastal island). One thing we cannot do is have holes in the "roof" to let the sunlight in below. This is due to technical limitations of the program that I don't want to deal with.
Some of the caves will obviously just be bare rock, but if civilizations are to live/thrive down here they'll need more than just rock. This underworld will mostly be populated by dwarves and goblins (I may add an even lower level full of daemons, but they don't really apply here).
Presumably they need some sort of “fertile” terrain similar to the farms on the surface, from which their civilizations would emerge and expand. Maybe mushrooms fields/forests could work? I know mushrooms get energy from existing biomatter, but maybe I can say that these get their energy from geothermal heat. Maybe then they could grow alongside lava flows the same way fields/forests grow along rivers? Any other ideas? They'll definitely be fishing the underground lakes, so maybe some kind of fish-farming next to the waters? Any other ideas for food-sources that could create the surpluses needed to build a civilization, or other underground terrain types that would look cool from a top-down strategic view?
I'm not looking for total realism here, just some believably and internal consistency. Thanks!