Having discovered that, on occasion, civs can build dungeons in cave sites I set out to go take a look.
My world has a couple of these rarities. The usual pattern is that kobolds build a market, kobold all get killed (statistics seen to indicate that building a market in your cave is just begging for extinction-by-megabeast). Then a more civilized bunch move in several years later and start doing weird things like building dungeons (and in one case, a keep).
My first quest was a failure. The Gutters of Slinking is an ex-kobold cave now inhabited by a tribe of wolf-people (mod civ, usually build towns). 3000 of them live here along with their many pets. Death by FPS upon approaching the cave. I guess 3000 wolf-people squished into a cave isn't what's expected. Ah well.
Next I headed out to Delightshafts. A jolly sounding cave currently inhabited by a tribe of wild humans. Only 125 of them, so easier on the CPU. At this cave lies the dungeon The Cloudy Murder-Night.
Approaching the cave I carefully took on the persona of a tribal wildman from the civ. I forget if the tribe in general are hostile to my civ, but best to take precautions. I am after all a stranger in their lands. And a minotaur.
The tribal people were fighting with their dogs when I arrived. Grabbing them, throwing them about, all jolly good fun and my disguise apparently worked as I was mostly ignored and able to observe their strange traditions.
The fun stopped when a pair of giant alligators suddenly appeared and tried to join in the scrap. Not wanting to be disturbed, I knocked them both unconscious and entered the cave (tribal people can fight the giant alligators later, best not to mess with the balance of nature when wandering in parts unknown).
Making my way to the bottom of the cave, I discovered, just as I suspected, that cave dungeons only exist in theoretic form, something to write about in books of Legend, but not for the casual wanderer to encounter.
Passing down the final ramp of the cave found me not in a dungeon, but unceremoniously dumped into an underground cavern lake. Surviving the swim I got lost in the caves and eaten by a giant cave spider.