I second the idea for the underground water sources acting more like aboveground, with the caveat that I'd like to see rivers flowing into lakes, great lakes, seas, and oceans.
Has anyone come up with any defined/refined ideas about some of the really bizarre monsters that might be living underground, in murky pools, or in oil? Since that's also been suggested.
I know these are the sorts of things that usually are vaguely handwaved at, for the creepyness/horror factor, but this being a computer game, that option really isn't there, in absolute terms.
The idea of a complex, yet really alien ecology, that would fit the nature of DF-containing top predators, prey, food sources, etc, and that would be able to contain some of the more stupendous and outlandish Cthonic horror type critters, in an atleast vaguely realistic (or just sensical) way-is quite appealing.
Maybe not in the shallower caves and tunnels, which might be pretty close to Earth, but in the really deep, deep depths where the unnamable squamous things play.
To give an example: I'm trying to develop my Protean species, which will be an intelligent civish species (doesn't have any hands, so can't really make much, but has other civ traits) that lives (or atleast spawns) in murky pools. They're an amphibious combination of a DF sea monster, a carp, and a cave catfish, with some borrowed traits from D&D Aboleths (one of the few original D&D monsters).
Basically in an effort to take the DF carp to it's nastiest extreme. So they'd be a top predator, existing in a very limited environment. Very slow moving on land though, so they wouldn't be able to just take everything over.