Utter Endurance: 'Iceman' and 'Ultramarathon Man'

incredible feats of enduranceABC News
Wim Hof swims in the frigid arctic waters of the North Pole.

What drives certain rare athletes to feats of endurance so astounding they invite the label "superhuman"? How are a select few men and women capable of exertion so far beyond their peers that new categories are needed even to describe what they do?

Watch the full story on "Superhumans!", a special edition of "20/20" Tuesday, June 1, at 10 p.m. ET.

For Wim "Iceman" Hof and Dean "Ultramarathon Man" Karnazes, the answer lies not in the freezing waters of the Arctic or endless desert dunes but somewhere inside. Both men have performed feats of such impossible endurance in such extreme conditions that they literally have become scientific curiosities.

Wim 'Iceman' Hof

North of the Arctic Circle, 51-year-old Wim Hof is running. The frozen tundra typically is reserved for polar bears, but Hof isn't running for his life. He is running for sport: 26 miles, half-naked, wearing just shorts and sandals in bone-chilling, minus-25-degree temperatures.

They are conditions that would kill any normal man -- but Hof is no ordinary mortal. Some say he's even superhuman.

"I'm able to control the body through just the power of the mind," said Hof, who calls himself the "Iceman." "Cold is merciless. It shows you where you are. What you are."

Hof's whole adult life has been dedicated to pushing the limits of human endurance. He's scaled Mount Everest, dived into the frigid waters beneath the North Pole and even turned himself into a human Popsicle -- immersing himself in ice for a record-setting 72 minutes, wearing only his signature shorts.

"It's almost beyond belief that someone, anyone, could live through that," said Dr. Ken Kamler, who specializes in extreme medicine. "He's immersed in ice water. And water will transmit heat 30 times faster than air. It literally sucks the life right out of you."

In an interview with "20/20," Hof explained why he isn't dead.

"I know my body. I know my mind. I know what I can do," he said. "I'm not afraid to die. But I'm afraid to not live."

Hof has been addicted to the freezing cold ever since he was a teenager with a self-described "irrational fascination" with a thin layer of ice on the water. Now, he spends winter nights bathing himself in the freezing canal near his home in Holland, in a sort of baptism by ice water.

"It's like cleaning the garbage within myself; the s***, the problems," he said. "Then I feel clean and alive again."

Wim Hof: Testing the 'Iceman'

Hof attributes his ability to withstand cold to an ancient Himalayan meditation technique called tummo -- meaning inner fire -- that he's practiced for years.

"[Tummo] enables [me] to heat up by concentrating my mind," Hof said. "It makes the body warmer. It's like pushing a heat button."

Kamler is familiar with the tradition of tummo.

"There are legends abound of practitioners of tummo sitting out on the ice naked except for wet sheets that they have draped around them, and they meditate," said Kamler. "The sheets dry and ice melts around them, even though they are in freezing temperatures."

Watch the full story on "Superhumans!", a special edition of "20/20" Tuesday, June 1, at 10 p.m. ET.

For most people, exposure to these levels of cold would be dangerous. Typically, when the body is put in 50 degree water, it begins to shut down in a selective way.

"It diverts blood flow entirely away from areas that are not necessary for immediate survival, such as the ears, the toes, the fingers, the nose," said Kamler.

The result is frostbite, then hypothermia. Once core body temperature drops beneath 90 degrees, the average person would die within minutes. But again -- Wim Hof is no ordinary guy.

ABC News decided to put Hof's abilities to withstand Arctic temperatures to the test. "20/20" correspondent Juju Chang joined the experiment.

First, Hof sat completely immersed in icy water. He seemed completely comfortable. After some time, Chang placed a foot in the pool, with pain and difficulty.

After a few minutes in the water, the dramatically different skin temperatures told the story. Chang's skin temperature was 81.3 degrees, while Hof -- despite having his entire body in the pool and being there for a longer period -- was nearly 10 degrees warmer, at 90.4 degrees.

Wim Hof: Ultimate Endurance Test

The ultimate test for Hof, however, was an Arctic marathon. He would have to battle the prolonged effects of exposure to cold. Could his mind maintain his body temperature while his body coped with the demands of running 26 miles in the snow?

Hof's wife, Carolina, feared that he was putting his life at risk.

"Finding another record, searching for peace, searching for the line between life and death, it's difficult for me," she said. "Noah is 6 and loves his father. I love Wim and we don't want to lose him."

Watch the full story on "Superhumans!", a special edition of "20/20" Tuesday, June 1, at 10 p.m. ET.

Despite his wife's fears, Hof believed his mind could trump the harsh terrain of Arctic Finland.

Experts estimated that a normal person would not last 15 minutes running in such conditions. Hof needed to keep his body moving to adapt to the icy temperatures -- but every step forced freezing air into his lungs.

"It is difficult to take a breath," said Arctic survival instructor Glyn David. "The air is so cold that your airway constricts. Skin will freeze at those temperatures -- it's as simple as that."

The temperature was minus 16 when Hof started -- and remember, he was only wearing shorts and sandals.

At first, Hof ran at a brisk and steady pace. But halfway through, he began to falter. After three hours of exposure to the glacial conditions, Hof's mental powers started to fade.

"It just goes out, the energy, and when the energy goes out, and then the cold comes in," said an exhausted and chilly Hof, as he continued to run.

After five hours, Hof couldn't run any further. But his slow pace increased the danger of damage from the cold.

"My physical exertion took all my energy. I could have collapsed then," Hof said.

With both body and mind stretched to the limits of endurance, Hof's behavior was becoming more and more erratic with each step. It seemed like Wim Hof's special powers would fail him this time.

"It's hard seeing someone you love pushing himself so far to feel life," said Carolina.

However, driven by the encouragement of his family, Hof finally was able stumble across the finish line. He had broken another world record.

"That's my reward -- the love of a woman and child," he exclaimed.

Dean Karnazes: 'Ultramarathon Man'

Just after midnight in his Monterey, Calif., hotel room the night before the Big Sur International Marathon, Dean Karnazes was already getting ready for the race. He checked the weather, grabbed a Raptor Bar -- an energy bar from one of his sponsors -- and headed for the starting line ... more than 30 miles away ... on foot.

For Karnazes, running a 26-mile marathon just isn't enough. By the time he hit the finish line in Big Sur, Karnazes had run 57 miles in all -- and that's nothing new for this seemingly tireless iron man.

Now 47, Karnazes has spent nearly the past two decades exerting his body in ways that would make even an elite athlete want to take a nap.

He's gone 350 miles without stopping; completed 50 marathons in 50 states in 50 days; run 135 miles in 120-degree heat through Death Valley; jogged to the South Pole in 40-below temperatures. He's run an estimated 100,000 miles in all. That's far enough to have won Karnazes the title "Ultramarathon Man."

"To me, my thing is just how far can the human body go," said Karnazes.

The open road is Karnazes's office these days. He has devoted his life to testing the limits of human endurance. But it wasn't always that way.

"I went to college, graduate school, business school, I had a cush corporate job," Karnazes said in an interview -- while he was running, of course. "That's kind of what was told to me that would make me happy."

A meltdown hit Karnazes at his 30th birthday party at a bar.

"I decided to run 30 miles that night to celebrate my 30th birthday," said Karnazes.

As he hit the road, Karnazes was harkening back to his days in high school when he ran cross-country. But at 30, he wasn't exactly super-fit.

"I hadn't done anything aerobic for over a decade. Walking up stairs, a flight of stairs, I'd be winded," he said.

Still, there he was, hours later, some 30 miles from the bar. A magnificent obsession had begun.

Karnazes did a 50-mile race, then a 100-miler. To keep his energy level up as he ran through the night, he'd have pizzas delivered to him on the road. He'd eat them without stopping. Truth is, he still hasn't stopped.

"I guess what I'm doing is a little bit extraordinary, and I just don't think of myself as being extraordinary. I think the human body's extraordinary," said Karnazes.

Dean Karnazes: Child Fitness

Karnazes' body is extraordinary in many ways. His resting heart rate is an extremely low 39 beats a minute. His body fat content is about 3 percent -- normal is twice that high.

Karnazes' non-running wife and their two children weren't surprised when he quit his job to run full-time.

While sponsorships and speaking engagements keep him going, Karnazes has also become an advocate for childhood fitness. He brings his own kid-like enthusiasm to his daily routine.

Watch the full story on "Superhumans!", a special edition of "20/20" Tuesday, June 1, at 10 p.m. ET.

"Perfect day is get up around 3:30 in the morning, bust out a marathon," Karnazes said. "Three or four o'clock go out for another quick training run, speed work. Maybe a 10-miler."

There's also cross-training, bike riding, sit-ups and push-ups.

But it's the grueling ultra-marathons that drive his body to the edge. Karnazes has passed out while running and suffered hallucinations after becoming dehydrated.

"I think that's where discipline comes in, mental discipline," said Karnazes. "I mean, it hurts so much, your body's saying stop and you're kind of overriding those mechanisms and forcing yourself to go on."

It helps that Karnazes has stayed largely injury-free. His endurance has led people like Dr. Jeff Shapiro of Sequoia Hospital in Redwood City, Calif., to test him and ponder how he does it.

"Dean trains his mind and his body to accomplish superhuman feats the same way everyone trains, by doing the activity over and over again," said Shapiro. "It's his passion for running."

It's a passion that, like Karnazes himself, shows no sign of stopping anytime soon.

"I'm actually getting stronger as I get older," Karnazes said. "I'm getting slower, but my ability to go further and withstand greater and greater amounts of pain seems to be improving."

Improving enough that Karnazes now imagines running a marathon in every country in the world.

But for Dean Karnazes, it's not about the finish line. It's about the running itself, and those moments when passionate exertion becomes pure exhilaration.

"Myself, as well as other runners and triathletes, found that when we're in pain and we're pushing, we're being tapped out, we feel the most alive," said Karnazes.