A few days ago a friend and I visited the Mammoth Caves in Kentucky. It's a national park and about midway between Louisville and Nashville, 90 mins in either direction.
It's the longest cave system in the world at more than 360 miles long. The rangers who guide the tours in the caves explained how the caves are formed by water flowing through and dissolving the porous limestone. What allows caves to form is overlaying sandstone, which acts like a roof to only let water through in a few areas.
Initially we were planning to go on the long tour, something like 4.5 hours and 4 miles but it was sold out when we got there. So we went on the New Entrance tour. Here's the new entrance:
That tour was so cool we elected to go on another in the afternoon, the Historic Tour, which goes through the areas of the cave that were used by the Native Americans in antiquity and rediscovered in the early 1800's. It's mostly dry cave.
The ranger mentioned that the slaves were the early tour guides and they made money that way. They also made money by selling materials for historic graffiti--antebellum grease smoke writing on the ceiling:
Above ground it was just under freezing. Below ground, it was comfortable sweat shirt weather in the 50's. Also you'll be dripped on.
Both tours I was one were rated half on their difficulty scale and beforehand in the orientations they stressed on knowing your limitations. The stairs and changes in elevation sound daunting. This gave me a small reservation especially since the first tour ranger said that one point near the bottom of the 200' initial descent was as narrow as his ranger hat was wide. However I realized that we had reached the bottom and I hadn't noticed anything bad; in a few spots you do have to be creative on the stairs with the railing. I suspect they write their descriptions for, say, a middle age office worker who isn't active. As an active young person I had no trouble.
I definitely want to get back and see more caves and park in nice weather. And it's the first time I've been to a national park that I can recall--I might have been through Shenandoah.
Closer to home, I can recommend a wet cave near Madison, Cave of the Mounds, worth visiting.