The last stop of our road trip was Chatanooga Tennessee. We passed it on the way up and made a note to stop here at Ruby Falls http://www.rubyfalls.com/) on the way back.
This 145-foot waterfall is inside a mountain and over 1000 feet below the surface. The caverns are limestone eroded by the water over millions of years. There is some info on the geology at Wikipedia https://en.wikipedia.org/wiki/Ruby_Falls).
To get here, you take an elevator down into the mountain and walk half a mile through narrow caverns. If you're not used to tight spaces, it can be a little claustrophobic, the thought of millions of tonnes of earth and rock above your head is unsettling. But our guide was kind enough to point out that in the event of a cave-in they had insurance and, we would all be covered.