On October 16, three cave divers slipped into the water at Eagle’s Nest, a remote sinkhole ensconced in pines in Chassahowitzka Wildlife Management Area in west-central Florida. From the shore it looks like an ordinary pond, but beneath the surface, it’s shaped like a sink with a long, rocky drain that descends into a system of underwater passages reaching 300 feet in depth.

Two of the men, Patrick Peacock and Chris Rittenmeyer, were experienced cave divers, and set off for deeper sections of the cavern, while their friend, Justin Blakely, explored an area closer to the surface. At 3 p.m., Blakely swam to their meeting point, but the others never showed up. He returned every half hour but there was no sign of his friends. By six o’clock, he called the police. After fruitless searches on Sunday evening, rescue divers found the bodies floating at a depth of 260 feet on the morning of October 17.

Cave diving is a notoriously dangerous sport, and in Eagle’s Nest alone at least 10 enthusiasts have died since 1981. The news prompted some locals to call for the cave to be closed to prevent further tragedy. As the Hernando County Sheriff’s Office conducts an investigation into the causes of death, many questions remain unanswered. Among them: What could possibly motivate divers to pass by a warning sign with an image of the grim reaper to explore a dark, watery underworld?

National Geographic Adventure caught up with one of the world’s best cave divers, Jill Heinerth, an underwater photographer and explorer-in-residence for the Royal Canadian Geographical Society, to uncover the risks, allure, and promise of this little-known sport.

View Images Two men stand at the surface of the Eagle's Nest underwater cave, where a pair of experienced divers died last weekend. Photograph by Octavio Jones, Alamy

Why do cave divers get into trouble at Eagle’s Nest?

Eagle’s Nest is deep, so you certainly have less time to deal with an issue in a deep cave. There’s a varied list of causative factors to accidents, everything from failing to run a continuous guideline [the lines that divers set to help them navigate back to open water], failing to reserve enough gas, equipment failures, medical problems, loss of visibility from disturbing sediment on the walls, floor, and ceiling of a cave. And of course people go above and beyond their training and experience. I’m not saying that any of these had any bearing on this accident because I just don’t know yet. There will be an in-depth investigation.

Some locals have been calling for a ban on access to Eagle’s Nest. What do you think?

It’s not generally the cave that kills people. I mean, if you’re driving down the highway, there are accidents all the time—but do you close the highway? If people have skiing accidents and get killed on mountains, do you close the mountains? It doesn’t really make sense. I understand that for people that are not cave divers, these environments are terrifying and make them feel claustrophobic, and they can’t imagine anything worse. But the exploration of such environments, the study of human physiology of what it takes to be a cave diver, all of these things are valuable to society. I do hope that any media reports will encourage people to have the right education before they enter these environments. There’s quite an extensive list of training that you need.

Still, in these mazelike caves, there’s a level of objective hazard that can’t be mitigated, right?

Sure, especially if there’s exploration involved, pushing the edge of the envelope. There’s always a risk. Some people call cave diving the most dangerous sport. That may or may not be true, but there’s a degree of risk that you’re always facing anytime you’re in a cave. The more complex the dive plan or the more unknowns, the more dangerous it might be. And a cave or diving environment that’s deep leaves you less time to deal with a catastrophic failure. In our list of people who have passed on in cave diving accidents, it’s everything from the inexperienced and untrained to the very experienced and trained cave divers.

Why would people take the risk?

For me, swimming in these groundwater environments is like swimming through the veins of mother earth and the lifeblood of the planet. It’s almost a spiritual reckoning. It’s an amazing thing. I mean, we know more about space than we do about inner space in these caves and springs. That fascination inspires people to want to explore, and in Florida there’s so much to explore. There’s so much to explore all over the world. I don’t know anything about the accident at this point, but I certainly understand the appeal that would have brought these divers to this remarkably beautiful place.

What is Eagle’s Nest like, where the divers were exploring?

It’s beautiful. It’s this almost perfectly circular pool in the middle of a low-lying swampy area. You go down a tubular chimney, dropping down the limestone, that’s really only the length of your body. Suddenly, at 70 feet, the floor and the walls all drop away and it’s just blackness all around you. You finally land down on top of this debris mound at about 130 feet, and you’re in this enormous room, like the size of an aircraft hangar. It’s stunningly beautiful.

Deeper in the cave, there are these beautifully scalloped walls where you can see how the water is carving the environment. There’s also the Super Room. I’m sure it’s called that because the first person who went there probably dropped their jaw and went, "Wow!" You come through this low passage and rise into this huge room that has colored layers of clay and deposition like a layer cake—bluish and white and gray and cream. When you look at those walls and those layers, you are looking at geologic time.

View Images A sign warns potential explorers of the Eagle's Nest cave's "life-threatening" conditions. Photograph by Octavio Jones, Alamy

Are there any signs of life?

We do see a little bit of life, like some catfish get into these caves. We also get cave-adapted animals that live their entire life cycle in the complete darkness of an underwater cave. Cave-adapted crayfish, for instance, are highly specialized animals that live in this very food-scarce environment, but they can live as long as 200 years. And they’re just pure snow white. You can see their internal organs inside their bodies. We have to wonder what these unique animals might teach us about evolution and survival.

Beyond recreation, part of the reason to dive in caves is for scientific research. What can we learn from these caves?

I’ve worked with climate scientists who are examining the underground geology. Astrobiologists are interested in algal mats and bacterial cultures because they might closely resemble life we might find on other planets. Biologists are interested in the cave-adapted animals, some of which have unique sensory organs that we don’t even understand yet. So there’s so much to learn. Caves are our groundwater and that’s the lifeblood that’s feeding just about everything topside, from agriculture to humanity to industry.

It sounds like there are many discoveries still to come.