Utter Endurance: 'Iceman' and 'Ultramarathon Man'

May 27, 2010 — -- 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.

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."