Thunderhead Falls is an old gold mine in the Black Hills of South Dakota.
Originally it was dry. However, when the miners got about 600 feet into the mine, they blasted open an underground water chamber. They were killed instantly. This water chamber is the source of the waterfall both inside the mine and outside.
The source of the water is the Rapid Creek on top of the mountain. The water from the mine rejoins Rapid Creek at the base of the mountain.
