Garrett McNamara might have surfed the tallest wave ever off the coast of Portugal, but he says he didn't even get an adrenaline rush from the experience. Uh, what?

McNamara, a pro big wave surfer known for extreme rides, set the Guinness World Record last spring for largest wave ever surfed with a 78-foot-tall wave at Nazaré off the coast of Portugal. In January, McNamara returned to Nazaré and surfed a wave estimated to be about 100 feet tall. He later told CNN's Anderson Cooper that he didn't even get a rush from it (video).

“Dude, you did not get a rush surfing that wave? Are you kidding me?" Cooper responded.

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“I am not kidding you, Anderson. I don't know what's wrong with me," McNamara said.

McNamara was precipitously close to a deadly cliff, he said his whole body was chattering and he nearly popped out of his straps. Even in the heady world of pro surfing, his utter lack of fear stands out.

Jaimal Yogis is a surfer I know whose recent book The Fear Project explores his own fears around big waves as well as the underlying science of fear. He has some ideas about what might have happened.

Yogis thinks McNamara likely exposed himself to so many big waves and survived so many wipe-outs that he's extinguished his typical fear response. The ancient fear center in the brain called the amygdala learns from experience, Yogis explained. “Garett's amygdala may be convinced, based on past experience, that he will survive any big wave because he has surfed so many successfully and he is still healthy," he told me.

There's also the dopamine, the pleasure chemical. Neuroscientists studying people who love dangerous sports have found that this personality type tends to have more dopamine flood their brains than average when they try something new and scary. Yogis thinks surfers like McNamara end up associating potentially stressful experiences with pleasure. This makes new big waves seem more enjoyable than frightening.

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Another factor Yogis suggested could be that while McNamara's big wave at Nazaré was impressive, most surfers who looked at the video and saw the wave's atypical shape knew that McNamara had surfed more difficult and frightening waves elsewhere. That probably made him feel more in control.

“His brain is primed to respond with very low stress in surf that might make the average surfer vomit or freeze up with fear," Yogis said. And he knows all about it. His book culminates in his firsthand experiences with Northern California's deadly Mavericks. My hands get sweaty just thinking about all this.

Photo: McNamara surfing near Portugal in January. Credit: Tó Mané via Time Magazine