It's an ability he discovered in himself as a young man 20 years ago.

"I had a stroll like this in the park with somebody and I saw the ice and I thought, what would happen if I go in there. I was really attracted to it. I went in, got rid of my clothes. Thirty seconds I was in," Hof said. "Tremendous good feeling when I came out and since then, I repeated it every day." It was the moment that Hof knew that his body was different somehow: He was able to withstand fatally freezing temperatures.

When he didn't experience frostbite or hypothermia, the body's usual reactions to extreme cold, his extraordinary ability started to get the attention of doctors who specialize in extreme medicine.