Devil's Den Prehistoric Spring

Devil's Den is a hole in the ground about two hours north of Orlando, near Gainesville, and it's one of the strangest places in Florida to put a tank on. You gear up in the parking lot, walk down a stone stairwell, and come out inside a cavern 120 feet across, with tree roots hanging through the opening in the roof and daylight landing on the water in a single shaft. The spring holds 72°F year round, so the same wetsuit works in June and in January.

The cavern is shaped like an inverted mushroom: narrow at the top, wide underneath. Once you drop below the platform the room opens up and you're finning over boulders and limestone ledges, with walls lined in fossil beds that go back 33 million years. Pleistocene fossils pulled from this spring are on display at the University of Florida's Museum of Natural History. Bring a light. The far corners of the cavern are dark, and the rock has more texture than you'd expect.

Every edge of the open-water zone is marked. The Stay Out signs sit at the limit of the recreational area, and the Grim Reaper stop sign guards the cave entrance beyond it. Everyone stopped for a photo and nobody went past it. The resident fish came out to look at us anyway.

We surfaced inside the cavern, sat on the platform between dives, and did it again. Then back up the stairs, one at a time.