One afternoon in March, a dozen barrel-chested young men stood on the frozen waters of Lake Päijänne, two hours north of Helsinki, Finland, cutting holes into two-foot-thick ice. Using large saws, hooks, and ropes, they managed, in an hour, to remove a half-dozen 5-by-6-foot blocks from the lake, each weighing about 200 pounds. Among the men were some of the world's best free-divers: Stig Severinsen, a tan 39-year-old Dane with a Mr. Clean head and a Greg Louganis body whose 14-liter lung capacity – more than twice the average man's – has enabled him to hold his breath underwater for a world-record 22 minutes; Guillaume Néry, a Gauloise-smoking French diver and the 2011 world champion in the "constant weight, no fins" deep-diving discipline; and Antero Joki, a bearded Finn who holds a national free-diving record. Though they joked about harvesting ice for vodka, they were readying the lake for a free-diving contest in the dead of winter, in a place where the water temperature hovers just above freezing.

There would be no world records set this day – no Guinness reps had been invited – and there were no championships on the line either. It was pure competition, a simple trial to see who olla munaa (had the balls) to go into water this cold and stay there the longest, and nothing more.

As they worked, I asked Joki why they didn't just cut the blocks and push them down into the lake's 300-foot-plus depths. Looked easier than hauling them out.

"Because sometimes the ice floats back into place after you get in, like a manhole cover," he told me, with typical Nordic reserve. "Then you're stuck."

The holes cut, the divers threw on wetsuits and began doing practice dives. Snow fell. Periodically, a magenta-faced figure in a wetsuit would emerge from the icy waters, saying something about the "cathedral-like light" and "eerie beauty" below.

Finally, around 4 pm, Severinsen got out of the water and removed his wetsuit, changing into the briefest of bright-blue Speedos. He grabbed a stuffed animal – a cat – that he carries around with him like a totem or a good-luck charm, and announced, "OK, Bøf, time to swim!" He sank into a lotus position on the lake ice. Severinsen is the author of the book Breatheology, a sort of breathing treatise and memoir, in which he describes his particular routine before a competitive dive or breath-hold: Four minutes prior to submersion, he sits by the water with his back straight and his eyes closed, breathing quietly, with "an inner smile and feeling light." At three minutes, he deepens his breath, inhaling and exhaling through his nose. At two minutes, he switches to heavier breathing and starts exhaling through his mouth, which he purses in an oval shape, a form of what he calls "purge breathing." The mouth acts as a valve, creating higher pressure in the lungs, which opens the alveoli and allows the blood to absorb more oxygen. After 30 seconds, he takes a long and luxurious yawn. Then, pressing his fingers lightly against his thighs (which, he says, causes the lung pressure to drop as his diaphragm shifts, again allowing the intake of more air), he starts to "pack," sucking in still more oxygen with his tongue, almost like a lion lapping water. He does this perhaps a dozen times, dons his diving mask, and then, still smiling, slips into the hole in the ice.Stig Severinsen grew up "Viking swimming" – basically, splashing around in ice holes – in the frigid lakes near his hometown of Ålborg, in northern Denmark. His first proper ice dive took place in Norway on his 37th birthday, in 2009, at the world's first under-ice free-diving competition (won by Néry). Unlike at Päijänne, the divers wore thick wetsuits, and there were international judges. "It was official," he says. "But at the end, I wanted something else, some extra fun."

So he jumped into the frozen lake wearing nothing but a swimsuit – did the backstroke, dove down 70 feet, and generally alarmed onlookers. Severinsen found this swim "fun" enough that he decided to do it again, a year later, when he challenged a decade-old ice-swimming record set by Wim Hof of the Netherlands, who, in March 2000, near the Finnish village of Kolari, swam 190 feet under the ice of a frozen lake. Severinsen had seen that feat on YouTube and wasn't impressed.

"He almost passed out," Severinsen said. "They had to drag him out of the water."

Severinsen, who has a Ph.D. in medicine and a master's in biology, doesn't earn his living from ice diving. But his cold-water exploits do bring attention to his other work: teaching highly paid, highly competitive corporate executives (and athletes, including champion cyclist Alberto Contador) who are looking for an edge, about the art and science of breathing. These lessons, at what he calls his Breatheology Academy, are not held in the frigid climes in which Severinsen often competes, but typically in a more relaxed, tropical location. There's good food, motivating and counterlogical conversation of the TED Talk variety, as well as swimming, meditation, and lectures on the benefits of better air intake. Severinsen contends that optimal breathing, with its focus on physical, mental, and emotional self-awareness, can be an effective weapon in the world of corporate warfare. Troels Hviid, a 37-year-old Microsoft project manager from Denmark, attended a weeklong seminar held on a boat in the Red Sea off Egypt. He uses the techniques he learned there doing deep dives to stay calm in pressured work situations. "You take a big breath and go down," he explained, "but the panic quickly grabs you, and I had to work on the mental side to stay calm. You have to generate a lot of positive thoughts to preserve oxygen." For this, and other instruction, Severinsen charges as much as $10,000 per week, per person, with recent sessions taking place in Las Vegas and San Diego.

Severinsen made his attempt on Hof's record in March 2010, in Denmark's Lake Knudsoe, 20 minutes from his home in Aarhus. His training for the feat included swims in the North Sea, which, because of the salt content, drops below freezing. Hundreds of Danes turned out, despite the cold, to witness Severinsen, in his now-signature Speedo, clip into a safety line, work through his breathing routine, and then slide into a square black hole in the frozen lake and swim beneath the ice until he'd reached another hole 236 feet away. The air was 2 degrees Fahrenheit. The water: 38. Severinsen spent 96 seconds in the water, and unlike Hof, he came out with no assistance.

Afterward, Severinsen, instead of warming up, took a victory lap on the ice, waving his towel behind him and playing to the crowd. Later, he would admit that the swim had been more difficult than he'd let on. "I'm not a crybaby, but it was very unpleasant and very noticeable," he said. "I'd never felt this kind of core cold." When asked how he beat Hof's record so easily, he said, "I'm not different from any other person. I just have the ability to shut down the sensory register for pain and discomfort."

Severinsen swam from one ice hole to another that day at Lake Päijänne, surfaced, and then went back. When he was done, he lingered on the ice, still only in his bathing suit, while the other competitors rushed into an aging sauna next to the lake. Later, Néry, to save face, braved the water like Severinsen, in only a swimsuit. This champion athlete – perhaps the world's best free-diver, someone utterly unafraid of blacking out underwater – was able to endure the cold water for all of 10 seconds. Severinsen stayed out there with Néry the whole time, laughing genially at his discomfort. Severinsen would receive no prizes, set no records, and do no victory laps. But it was clear that in this contest, and perhaps in any contest of this sort, Severinsen had triumphed.