Devils Den Spring is one of North America's most prehistoric places. This underground spring inside a dry cave has been home to many extinct animal fossils dating back to the Pleistocene Age, which is on display at The University of Florida's Museum of Natural History.
Devil's Den is formed by a karst window, in which the roof over a subterranean river has collapsed, exposing the water to the open surface, near Williston, Florida. It is privately owned and operated as a SCUBA diving training and recreational facility. The water in the underground river is a constant 72 °F degrees.