Subtropolis

I used to go caving in parts of OH, TN, and KY back in my 20's and 30's.

Some of the "wild' caves I was in were truly amazing.

One had a room that was as big as a football stadium with a rock the size of a garage in the middle.
Attached to the rock was a plastic tube with some sheets of notebook paper containing names of people that had been there.
I was #24.