You've already seen Tennessee's natural wonders, in the form of magnificent mountains, rivers and lush forests. But there's more to the Volunteer State than meets the eye. The rolling hills hide miles of caves, formed over millennia as the acid in groundwater eats away limestone. Cumberland Caverns is one of the state's longest caves, and holds fascinating and eerie displays of underground beauty. Like the spectacular flowstone “Cathedral” -- a huge chamber formed of liquid-looking calcium deposits, the Crystal Palace with “flowers” of sparkling gypsum, the narrow tunnels known as the Devil's Backbone, and the enormous Hall of the Mountain King (one of the country's largest cave rooms). Scenic walking tours are held multiple times a day, and for the non-claustrophobic, try an overnight spelunking trip. Or attend one of their weekly "Bluegrass Underground" concerts.