To many vacationers, a Florida sinkhole has about as much appeal as a highway detour. Although the state's landscape is dotted with thousands of such depressions—underground caverns that have collapsed and often filled with water—most are nothing more than small ponds or swimming holes. About a mile off Highway 41 near Venice, however, one such sinkhole has long been a mini-mecca for health-seekers in search of mineral baths. Known as Warm Mineral Springs, it is, according to some enthusiasts, the legendary fountain of youth that Ponce de León grew old trying to find.

Archaeologist Wilburn "Sonny" Cockrell, 47, wouldn't know about that, but he can vouch for the spring's powers of preservation. Cockrell has made innumerable dives into the pitch-black, 230-foot depths and come back with a variety of ancient bones and fossils, including the remains of an 11,000-year-old Paleo-Indian hunter that he found on a ledge 40 feet down. Almost singlehanded, he has made the sinkhole one of the most significant underwater archaeological sites in the world.

Like most such pools, Warm Mineral Springs was a cave until its ceiling fell in during the ice age 30,000 years ago. Unlike most others, though, it is fed daily with 20 million gallons of mineral-rich, 92-degree water that gurgles into it from an underground spring. Below a depth of 20 feet, the absence of oxygen and the presence of the minerals create a nearly perfect environment for preserving organic materials.

As the water level rose through the centuries, animals and people dropped or threw things into the pool and sometimes died or were buried on a ledge that was then above water. Today, Cockrell says, there is a cone-shaped mound of debris rising 106 feet from the bottom, and layer by layer, it holds an uninterrupted archaeological record. At the bottom of the mound are the remains of long-extinct animals and early humans. Higher up, says Cockrell, are "everything from bunion pads and Band-Aids" to an aluminum lounge chair, awaiting the interest, perhaps, of archaeologists in some future millennium.

From the pool, Cockrell has retrieved the bones of saber-toothed cats and giant ground sloths, as well as fossilized alligator feces and unpetrified wood that has survived 12,000 years of immersion. Down there, Cockrell says, "There are actual tree leaves, acorns, hickory nuts—even though hickory trees have not flourished in this region for 8,000 years."

To date, Cockrell's prize find is still the 11,000-year-old skeleton of the hunter, which he retrieved in 1973. This prehistoric man, who had a ridged head, shovel-shaped front teeth and stood no more than 5'2" when alive, was found curled in the fetal position and tucked deep in a stone crevice. Two broken-off stalactites had been pushed in front of him as if to seal his tomb, thereby providing the earliest evidence of an intentional burial ever discovered in North America. Buried with the body was a piece of a spear-thrower—a tool made of shell that enabled a hunter to fling his weapon farther and more forcefully. It is "one of the most important inventions ever," marvels Cockrell. With it, humans could "kill large animals from a distance," he says, and "for the first time have large, stable families because they could kill more food."

Cockrell's descents into the pool began 16 years ago, but his immersion in archaeology had its start long before. Born in Missouri, he was 4 years old, with an infant sister, when his father was killed in World War II. His mother remarried, bore five more children and moved with the growing family to Alabama when Sonny was 12. There, while a sophomore at the University of Alabama, he went on a geology-class dig that unearthed a 66-million-year-old whale fossil.

Soon Cockrell had a degree in anthropology, a wife and the first of two children on the way. He began teaching high school English and social studies, first in rural Alabama, later in Florida. Then, in 1965, a former anthropology instructor got him a job as Florida's state-highway archaeologist, charged with insuring that road crews didn't pave over valuable archaeological sites. "I had a four-wheel drive, a pair of big snake boots and the run of the entire state," he recalls happily. Better still, he also had the means to pursue a master's degree at Florida State University.

In 1972, Cockrell, a recreational scuba diver, was appointed the state's underwater archaeologist. In that post, he says, "I came to be known as the Great Satan to treasure hunters." To this day, he regards any careless digging for sunken gold or artifacts as something "worse than the slaughter of whales or the pollution of the environment. If you stop killing whales, the species will come back," he says, "but you can never replace an archaeological site." Alas, state legislators weren't convinced and in 1983 abolished his job.

By then, Cockrell had already explored Warm Mineral Springs, and now he became a one-man lobby to preserve the site for scientific research. He lived off unemployment insurance, mortgaged his Jeep and borrowed from friends to keep his project going. Finally, with help from State Senator Bob Johnson, he managed to get annual state grants totaling $300,000 routed through Florida State University, which now employs him as a research director in anthropology. But more money is necessary, says Cockrell. An underwater trench dug in the debris is in danger of collapse without costly shoring, and mixed-gas diving equipment is needed in order to work safely at those depths.

Cockrell's own deep dives have caused episodes of the bends as well as nitrogen narcosis, a temporary malady resulting in impaired judgment from too much nitrogen in the blood. The intense pressure has also cracked several of his teeth and diminished the hearing in his right ear. Even so, Cockrell presses on, working by day with fellow divers Steve Koski, Skip Wood and his still-friendly former second wife, project manager Barbara O'Horo. At night he retreats alone to a modest duplex about two blocks from the site. "I often ask myself, 'Why do I do this?' " he admits. "It's so black down there it's easy to get lost, and sometimes I'm so terrified I have to come back up. But then I start making out the cavern walls and the things that are there, and I want to stay. It's like being in a dream."

—Dan Chu, and Meg Grant in Warm Mineral Springs