It occurred to me as I was looking at some of the dungeon maps that I have actually been in caves and explored the "underworld". I was curious if other folks have done so as well, and if they have any caves that they consider their favorites.

My personal favorite are the Kartchner Caverns in Arizona.

http://www.pr.state.az.us/Parks/parkhtml/kartchner.html

One of the few 'living' caverns in the US. They have it hermetically sealed and very strict rules on touring and such. All the other caverns I've been in have been fairly devoid of life and dried out.