THE WHALE DISAPPEARS before Kirkconnell gets close, but it's clear that this area draws very large animals. After 30 minutes of drift diving, Clasen swims to the boat and says quietly, "We're going to need more chum."

Kirkconnell, a flurry of energy, launches out of the water, over the gunwale, and into the boat. "What?! What did you see?" he cries.

"Tuna," Clasen says. "Six feet long. Three or four hundred pounds." A claim like that would normally warrant skepticism, but Clasen knows big fish as well as he knows these waters. He says it's likely a bigeye tuna.

Tuna are these spearfishermen's dream catch. The bluefin is the real king, a 15-foot, 1,500-pound torpedo that can hit speeds of 60 miles per hour. In 2001, a bluefin fetched more than $173,000 at a Japanese fish market, and voracious demand drives unsustainable levels of global fishing. According to the National Marine Fisheries Service, the U.S. stocks—which spawn in the Gulf of Mexico—are overfished. In 2007, the industry could land less than 15 percent of its quota. Even so, commercial and recreational take of bluefin continues, though spearfishermen are prohibited from hunting the animal in the Atlantic and the Gulf. Clasen had to fly to New Zealand in 2007 to hunt bluefin and came away with a 580-pounder.

The bigeye Clasen just spotted is legal to hunt, and there's not a second to waste. Clasen and Head quickly add a pair of buoys to the two already clipped to the gun by a long cord. The extra floats will help them track the fish from the surface if the quarry runs after being hit. Kirkconnell furiously cuts the bonito for chum. "I'll turn this boat into a processing plant," he promises, throwing slider-steak-size chunks of meat into the water. Clasen spends several minutes hanging off the stern, taking slow, deep breaths, then slides into the current and disappears.

Forty-five seconds later, Clasen's buoy rises like a big orange exclamation point, signaling that he's stretched out his 100-foot bungee line. "This is when I worry about him," Kirkconnell says. He knows all too well how quickly things can go wrong.

Last July, Kirkconnell went diving 70 miles off the west coast of Florida with Steve Bennett, a 20-year-old University of Florida student. Bennett suffered a shallow-water blackout on his way up from 75 feet, and by the time Kirkconnell saw him plummeting to the bottom, Bennett was out of reach. Kirkconnell aimed his speargun at Bennett's thigh in hopes of burying the shaft in his friend's leg and pulling him to the surface, but he couldn't get a clear shot, so he aimed for the fiberglass blade of Bennett's fin. Miraculously, it held, but by the time Kirkconnell pulled Bennett to the surface, he'd already been underwater for four minutes. His face had turned blue, and he bled from every orifice in his head. Kirkconnell began CPR, and a Coast Guard helicopter flew Bennett to the hospital. He made a full recovery, and after five days the hospital released him. "Cameron is a hero," lauds Bennett, who went diving a few weeks later.

"That was the best shot of my life," says Kirkconnell.

With Clasen's hover at more than 100 feet going into the second minute, his body calls upon a physiological adaptation millions of years in the making, the mammalian diving reflex. It begins with a process called bradycardia. Though Clasen is essentially running a 40-yard dash while holding his breath, his heart slows to half its average resting rate, helping to conserve oxygen. His arteries constrict—a blood shunt—and funnel blood to the heart and brain rather than the extremities. And the spleen, an organ better known for its role in the immune system, releases extra red blood cells, adding more oxygen to Clasen's rapidly dwindling stores.

Clasen finally surfaces empty-handed after 1:55 underwater. Kirkconnell pulls him in by the safety line hanging off the back of the boat. Clasen is enervated by the hunt, disappointed by the result, but excited by the prospect of finding monsters like this again. "Tuna is like gold," he told me earlier. "If there were no more tuna, it would rip my soul out."