Devil's Den is a solution sinkhole near Williston in Florida: a small opening in the ground leading down into a large cavern that is partly filled with water.
The name comes from steam. The underground river is geothermally warmed and holds a constant 72 degrees Fahrenheit, so on cold mornings water vapour rises out of the opening in a visible plume. To early settlers that looked like a chimney venting from Hell, and the name stuck.
The shape below is stranger than the entrance suggests. The cave widens underwater into what has been described as an inverted mushroom: about 100 feet across at the water surface, and up to 200 feet across below it. The original opening was a small sinkhole that visitors had to squeeze through, and it was enlarged in the 1990s to make access easier. The water level inside has fallen along with the water table in the surrounding area.
The entrance is a karst window: the roof over a subterranean river collapsed and exposed the water to daylight. The window is far smaller than the cavern below and may have formed geologically recently. Four underwater passages run off the pool, between 5 and 90 feet down as surveyed in 1975, and the water is over 20 metres deep.
The science happened in one burst. Palaeontologists made at least 96 dives here in 1960, the only time the site has been studied extensively. Chamber 3, 70 feet under water, held animal remains, human remains and artefacts together. The animals were Pleistocene and the list is long: 47 mammal species including mastodons, ground sloths, camels, horses, dire wolves, bog lemmings, Florida spectacled bears, sabre-toothed cats and peccaries, along with many fish, amphibian, reptile and bird fossils. The mix of species suggests the land above the cavern at the end of the Pleistocene was mesic forest ringed by dry savanna.
The cave was probably dry then, when sea level and the local water table were very low, which is how everything got in. As of 2008 the human remains had still not been systematically studied.
It is privately owned and run as a scuba diving training and recreation facility, which is an unusual afterlife for a palaeontological site, and the constant water temperature is exactly why divers use it year round. Snorkelling needs a reservation, no exceptions, and diving needs at least Open Water certification. The way down is a narrow, steep wooden staircase that gets wet.
Other places on the map within a couple of hours.
