An interesting place I'd never visited before is the Lost Sea in Sweetwater, Tennessee, about an hour or so south of Knoxville. This place is listed in the Guinness Book of World Records as the largest underground lake in America. The history of humans using the cave dates back to the Cherokee Indians, but there have been fossils dating more than 20,000 years old found within the cave's many crevices and caverns. It was likely used in Cherokee ritual, as storage for vegetables in the 1800s, as a source of materials used for gunpowder in the Civil War, as a location for the production of moonshine, and then as a place for dancing and cockfights in the early 1900s. (Source: "The History of the Lost Sea.").
Now it's open to visitors to tour, and I went with a group of friends (some of whom were actually tour leaders themselves when they were in high school), to see this place first-hand.
After walking for a while, you get into little boats and they drive you out into the dark cavern on the water. It's kind of scary, but definitely a unique experience!
Britt and Jen in the boat.
Joey and Megan in the boat.
Leaving behind the lights for a trip on the lake.
A moonshine still.
I found it amazing that these cute little ferns were growing underground, with no light except the man-made lights added for tours.