Devil's Den Spring, located in Williston, Florida, is a privately owned prehistoric spring and cave system with an average water depth of 50 to 55 feet (15–17 meters). The spring is situated within an underground dry cave with a "chimney" opening at the top that allows sunlight to filter down into the crystal-clear water. The surface of the pool has a diameter of approximately 120 feet. Because the water remains a constant 72 degrees Fahrenheit (22°C) year-round, steam often rises from the opening on cold winter mornings, giving the site its "devilish" name. Access to the water is restricted to snorkelers and scuba divers only; casual swimming is not permitted to preserve the delicate ecosystem and ancient fossil beds, which contain remains dating back 33 million years. Snorkelers generally stay near the surface to admire the rock formations and schools of fish, while divers can explore the deeper reaches and small swim-throughs at the bottom. It is a world-class training center for cave diving, but all participants must be at least 18 years old or accompanied by a guardian to enter the "Den."