FROM THE SURFACE, the oil platform Medusa appears an unlikely fishing hole. The rig, a tight weave of steel girders supporting cranes, a helipad, and the roughnecks who run it, rests atop a narrow support pillar like a giant industrial lollipop. Thirty-six miles south of the Mississippi River's mouth, off the coast of Louisiana, in more than 2,200 feet of water, Medusa extracts up to 40,000 barrels of crude oil and 110 million cubic feet of natural gas every day. In one of nature's ironic twists, this floating monolith doubles as a thriving, vertical coral reef, which is precisely the reason that Craig Clasen and Cameron Kirkconnell motored a crew out there one sunny day last June.

As two of the world's best spearfishermen, Clasen and Kirkconnell, both 32, are famous in diving circles for spearing fish the size of offensive linemen. But unlike many of their peers who hunt using scuba gear, Clasen and Kirkconnell are freedivers. In one of the purest, most physically challenging forms of hunting, the men dive to 100 feet on a single breath for two minutes or more, a discipline Kirkconnell describes as "calculated insanity."

After hours of yo-yoing through the water column at Medusa, the men had a large cooler stocked with an Audubon guide of game fish: wahoo, tuna, dorado. They planned on eating well that night. Everything was going according to plan, until suddenly it wasn't.

As the crew prepared to leave, a third man, filmmaker Ryan McInnis, became distracted filming a playful pair of squid at the surface, 150 feet from the boat. When he turned around, the 33-year-old McInnis saw a 12-foot tiger shark, drawn by a perfume of bloody chum. The shark charged. Armed with only his video camera, McInnis knew he had to do something, so he pressed RECORD. The shark veered away just a foot from him and began to circle. McInnis yelled for the boat.

With a tangle of lines hanging off the stern, Kirkconnell couldn't immediately speed over to McInnis. Clasen (nickname: "Ragin' Cajun"), still in the water and with just seconds to act, swam toward his friend, speargun in hand.

By the time Clasen reached McInnis, the shark had tightened its circle, and the men couldn't reach the boat and fend off the predator simultaneously. Clasen has spent much of his life swimming harmoniously with sharks, but this one had a different feel. "Every bone in my body was telling me that this shark was up there to feed," says Clasen.

Tigers are swimming garbage disposals—they've been known to swallow sea turtles and old tires whole. When the shark made a move toward the men, Clasen shot it through the gills.

It was a devastating blow but not lethal, and the hunter's code that governs Clasen compelled him to finish the job. But evolution designed these animals masterfully—big body, small brain—and Clasen couldn't "stone" the shark, despite shooting it several more times. Every time he approached the fish underwater, it would snap violently to life, until finally he took a deep breath of air, swam down to 40 feet, slid under its belly, and wrestled the shark to the surface by its pectoral fins. "I had to bear-hug it to keep it from biting my head off," Clasen recalls. Kirkconnell threw him a rope, which he lassoed around the shark's tail. They dragged it behind the boat until it drowned.

A somber mood fell over the men. "I want to be very clear," Clasen said remorsefully. "That was not a proud kill for me, but I didn't have a choice. She was just being a shark."The men eat what they kill, so they cut a fillet off the tiger and ate it sashimi style. It tasted like oatmeal with rubber bands in it. "It was terrible," Clasen recalls.

After hearing about the story from photographer D.J. Struntz, who was in the water when it happened, I gave Clasen a call. "A lot of people don't understand what we do," he said. "They're going to think I've lost my mind. But this is not Disneyland. You come on down here. We'll show you."