Map gen and bug/aquifer dickery can cause visibly-bottomless caverns and cave rivers respectively:
- For very deep pits that are only bottomless-by-vision, generate a map with 100 zlevels between cavern layers. Very simply, this makes it impossible for a dwarf or tame animal standing on the edge to see the bottom of the pit. Sadly if the dwarf or tame animal gets flung off the edge and falls, they'll see everything all the way to the bottom, ruining the visual effect.
- For "cave rivers" what's required is to generate a world with a very shallow first cavern layer, and then embark on a site with a very deep aquifer. Hopefully, the aquifer layer will intersect the first cavern, creating aquifer-lined cavern walls that will dump water into the cavern, creating a flowing or at least a trickling "river". Alternately, hope for a map gen bug that causes a map-edged lake high in a cavern to lack a containing wall, causing the lake to spill its water downards to the lower areas of the same cavern... But to truely get a cave river in the current version, you'd have to generate an aquifer-lined cavern 1 that had a downward passage or pit to cavern 2 in the path of the water.
... Though, i too wish that functional cave rivers and the "chasm" tile existed again. But then, those kinds of features would function more properly when Toady works on the multi-z level mineral viens; once multi-z viens happen, i'm sure it'd be easy to just tell the game to hollow out the space instead of filling it with ore, and then fill it with water/magma instead.
Of course technically, truely bottomless chasms should be possible right now anyways; it'd just require generating the bottomless chasms LAST, over-writing the hell tiles' "glowing pits" and replacing it with the "chasm" tile we all knew and loved, and of course, creating a wall of semi-molten rock or slade, seperating the horrors from the rest of the world.