Lechuguilla Cave is part of the Carlsbad Caverns Natural Park in New Mexico and is regarded as one of the most beautiful caves, with some of the most unique geography, in the entire world.
You can't visit.
Because of the delicacy of many of the formations, the cave is only open to scientists and the explorers who are still figuring out what all is down there. Nobody else is allowed in. Or, rather, nobody else but David Attenborough.
This video from the Planet Earth TV series takes you down into Lechuguilla for some amazing sights and fascinating commentary on the chemistry and biology that make this cave so strange and lovely. Even more impressive, nobody knew it was there until 1986.
Psst, Nova has a whole page on Lechiguilla, if you want to read more.
I love that sentiment expressed at the end, that there may yet be many more such subterranean wonders beneath our feet. I also would like to know more about "extremophile bacteria feeding on the very rock" - could their processes be sped up and controlled to create planned caves in a less-than-geologic timescale?
Probably not, but its fun to imagine voracious colonies of lithoautotrophic bacteria carving out new chambers, selectively killed by spraying bleach or some other deterrent or fostered by spreading the dust of their favorite mineral compounds in the direction the cave engineer hopes they'll grow. Imagine if such a colony could grow cave systems on the order of feet per day. Let's limit their behavior to keep from excessive cave-ins by imagining that they leave behind directional cave systems that gradually wind their way down into the earth, with occasional branches but not expansion in every direction. In other words: they create lairs.
Warning: I am not a chemist: What will the bacteria leave behind? Well, as limestone is primarily CaCO3, they'll be breaking it up and creating gorgeous, semi-organic-looking calcium deposits, and it'll be releasing carbon dioxide and oxygen into the air. Would lithophagic bacteria leave behind breathable cave systems? Let's imagine so. Let's also say they're of value to miners, because if you point them at a vein of rock with your favorite precious metal in it, they'll leave it lying around as they pass through it and you just sweep it up in their wake.
In an even more fantastic setting, you could have the equivalent of Gelatinous Cubes, but these 10' cubes eat inorganic matter instead of organic stuff. They excrete a solvent for rock, but only on one side of the cube at a time. Unfortunately they are impossible to control or predict, but their likely movements have carefully been modeled as a particular set of random dungeon-generating tables! All you have to do is add doors, denizens, and treasure! It's a bit over the line from where I like my fantasy, but if you're in a pinch to justify miles and miles of endless dungeon that no sentient being in his/her right mind would ever create (see how I carefully excluded us all there?), you might need rock-eating-cubes doing all your dungeonbuilding for you.
If you do have such things actively building in an underground setting when delvers, adventurers, or other denizens take up residence, it might be fun to track where they are and where the expansion is happening. Like the miners who sweep up gold dust in the wake of our lithoautotrophs, it might benefit a party to hang behind an excavator cube as it works, waiting to see if it connects them to an otherwise hard-to-reach part of the delve (while you roll dice on the 'excavator-cube-movement' [aka random dungeon] table to see what it does).
Or maybe you, a dungeon designer and builder, need to hire some delvers to go deep into the mazes created by such things in order to have them harvest autolithotrophs to bring to your nascent dungeon to get it started or help with that wing you've been meaning to expand. You'll have to pay them well, and I recommend not telling them the actual location of your underground construction lest they come and raid it later, but if you're looking to get your hands on these kinds of creatures, you don't really have many more options than to have those loathsome adventurers go on an underground retrieval mission for you.