Found a little bit more time today (still far from finishing the game, though), mostly improved geology:

• On non-ground terrain tiles (i.e. walls, or any former ground-terrain onto which feces were dumped), fluid will trickle down. The earth will soak up the fluid, and emit it as wetness on ground terrain tiles
• Wetness on a tile may stack up temporarily, but any such stack may collapse quickly, excessive wetness being distributed to less wet ground terrain tiles.
• But the fewer ground terrain tiles are available, the greater the tendency to stack up wetness, i.e. towards higher fluid concentrations on individual ground terrain tiles.
• The more fluid concentrates on a ground terrain tile, the greater the chance that this tile will erode into a hole, to fall into which will be deadly.
• Re-wrote large parts of the internal representation of terrain height and wetness, made the mechanics based on these more consistent.
• As always, still playing around wildly with how to color or symbolize different things on the game map.

Today's screenshot: