Cherokee Caverns, Knoxville

The Cherokee Caverns first started forming 300 million years ago. Today, their stalagmite- and stalactite-filled underground chambers are maintained by volunteers who open them to the public several times a year for special events, including Sunday night concerts, a Halloween trick-or-treating event, and Christmas in the Cave. (The holiday celebration makes the most of the cave's comfortable, permanent internal temperature: 58 degrees.)

Below, some landmark moments in the caves' transformation from loose shells, sand, and clay into a modern-day social hub.

• Pre-1854: Native Americans discover the cave, as indicated by ancient torch-marks on the cave walls.
• 1854: A local farmer, Robert Crudington, rediscovers the caves. He noticed an eerie fog rising from the rocks covering the cave mouth.
• 1929: Crudington's daughter, Margaret Gentry, hosts the first public tour of the caverns.
• Between 1947 and 1960: Homer Harris, known as the world's tallest singing cowboy, holds a Western music show in the caves with his trick horse, Stardust.
• 1960s: A chef starts a restaurant in the cave mouth. Showing great self-restraint, he does not call it "Mouth Food."
• 1980: The restaurant burns down, seriously damaging the cave and ushering in an era of neglect and vandalism.
• 1990s: Volunteers take over the caverns, turning them into the event space and geological education site they are today.