Visit USA: information about Thunder Head Underground falls
(600 feet inside a mountain)
Buried deep within 600 feet of mountain, a gushing 8 cubic feet of water falls vertically for more than 30 feet. This unique attraction is Thunderhead Underground Falls, located 10 miles outside of Rapid City off Highway 44 West in the central Black hills.
This hydro-spectacle dates back to just after the Custer Expedition of 1874, which means Thunderhead is one of the earliest operating gold mines in the Hills, and the oldest gold mine in the Black Hills open to the public. (On the scenic route to Mount Rushmore)