This is the Devil’s Den near Williston, Florida.
The natural structure is known as a ‘karst window’, a geomorphic feature formed from the dissolution of carbonate bedrock where a spring emerges then the discharge abruptly disappears into a sinkhole.
From the surface, it looks like a trap door opened up to a mystical realm of water and light below.
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 (22 °C) degrees. In cold weather water vapor rising from the surface of the river forms a visible plume above the entrance to the cave, which suggested a chimney from Hell to early settlers. The opening to the surface was originally a small solution hole, through which visitors had to squeeze to reach the water. The opening was enlarged in the 1990s to ease access. The cave expands below water level (a shape described as an “inverted mushroom”) to up to 200 feet (61 m) across. The water level in the cave has fallen along with the water table in the area. The cave was opened to the public as a dive site in the early 1990s.
Four underwater passages extend from the pool under the opening, from 5 feet (1.5 m) to 90 feet (27 m) under the surface of the water. The passage called chamber 3, 70 feet (21 m) under water, contained animal and human remains and artifacts. The animal remains, which appeared to be associated with the human remains and artifacts, were from extinct (Pleistocene) species, including mastodons, ground sloths, camels, horses, dire wolves, bog lemmings, Florida spectacled bears, saber-toothed cats, and peccaries. The human remains have been dated to about 7,500 BC.