THERE'S HARDLY a more egalitarian sport than freediving: Trudge into the water, put on a dive mask, take a big breath, and kick toward the bottom. Yet rarely is it that simple, and in recent years the sport has developed a nasty habit of killing its stars.

In 2002, 28-year-old Frenchwoman Audrey Mestre died while attempting a "no limits" free­diving world record in the Dominican Republic. A weighted sled pulled her to 561 feet, but the balloon meant to shoot her to the surface never fully inflated, and she drowned. In 2007, former world champion Loïc Leferme, also from France, died during a no-limits dive to the same depth. While the world's best freediving spearfishermen don't use sleds or balloons or go anywhere near those depths, they nevertheless account for most of the sport's fatalities.

Shallow-water blackout is the main culprit. As the lungs compress under pressure at depth, they push oxygen into the blood and tissues. When a diver ascends, however, expanding lungs suck oxygen out of the bloodstream and tissues, increasing the chances of extreme oxygen deficiency and blackout. Intermediate divers are often skilled enough to dive deep and stay down for extended periods, but they can lack the lung capacity to complete the round-trip. They are the ones most likely to black out during ascent and drown. But even experts can get into trouble. In 2004, champion Hawaiian spearfisherman Gene Higa drowned off Oahu, likely as a result of shallow-water blackout.

On the oil platforms, blackout is but one of myriad dangers. The structures Clasen and Kirkconnell prefer sit in hundreds, if not thousands, of feet of water, where strong currents can yank a diver toward Belize. Sharks are an omnipresent threat, as is falling debris from the platforms hundreds of feet above.

The quarry itself can also kill. A line connects the spear to the gun's barrel, and even a 30-pound fish, when shot, can tie a diver in Boy Scout knots around a pylon in seconds. Then, of course, there's the structural engineering to consider. Thrusters power some platforms, and a diver who swims too close runs the risk of becoming ground beef in a giant propeller. In 2007, 41-year-old former Navy SEAL James Martin drowned after shooting a big fish, which slammed him against the rig and knocked him unconscious.

People have been "diving the rigs" for decades—it's legal, and the roughnecks and divers have forged a peaceful coexistence. Clasen, a fifth-generation New Orleans resident, began when he was 16. At six foot two, he's built like a tank and speaks with an aw-shucks southern drawl that belies the fact that he's been successful at nearly everything he's done.

At Isidore Newman School, Clasen was an all-state linebacker, and his senior year he became co-captain, taking over for a quarterback by the name of Peyton Manning. After graduating from the U.S. Merchant Marine Academy, at Kings Point, New York (where he met Kirkconnell), he fulfilled his military service by piloting supply ships to Kuwait in the early days of the Iraq war. Today he lives in a slick French Quarter bachelor pad and, like his father and grandfather before him, is a Mississippi River pilot, one of the most coveted jobs on the water.

Kirkconnell, six-three, brash, and outspoken, with thick, unkempt brown hair, holds five spearfishing world records, including a 201-pound dogtooth tuna—a ferocious, rocket-fast deep-water fish widely considered the sport's most difficult quarry—he shot in Indonesia. He's licensed to captain just about anything that floats, and recently he's been working on bulk carriers as a first mate, spending months at a time crossing oceans. During downtime at sea, he stays in shape by running a ship's 14 stories while holding his breath.

I'm a longtime surfer and dive master, but I held no illusions of keeping up with these guys. I hoped just to stay within sight. To that end, two weeks before meeting Clasen and Kirkconnell, I took a four-day course in Monterey, California, with Performance Freediving. Instructor Kirk Krack established the importance of proper training early on, when he asked the class, "What other sports do you know, besides spearfishing, that [annually] will have 40 to 50 fatalities in such a small group, except competitive Russian roulette?"

By the end of the first day, Krack and his wife, seven-time freediving world-record holder Mandy-Rae Cruickshank, had taught me to extend my feeble 90-second breath hold to three and a half minutes, facedown in a pool. On the last day, I dove to 82 feet in Monterey Bay. The trick, as Krack had explained, was mental. Every time I passed the two-minute mark in the pool, my diaphragm spasmed in an effort to stimulate breathing. It felt like a mix between severe hiccups and someone trying to rip out my trachea, but this was simply a natural reaction to the buildup of carbon dioxide, not a dearth of oxygen. Even as my body began to twitch after three minutes, I still had enough oxygen to continue, as long as I could disregard the physical discomfort. Still, Krack warned, it's not a sensation to ignore for too long.