It's hard enough to scale Mount Everest. It's even harder to reach its summit without an oxygen boost.
While 4,000 hikers have reached the top of the world's tallest mountain since 1953, fewer than 200 have accomplished that feat without supplemental oxygen.
That's what two professional hikers, explorer Adrian Ballinger and photojournalist Cory Richards, recently attempted.
"This was something I'd dreamt about since I was a kid," Ballinger told CNBC's "On The Money" in an interview.
Both survived. Yet only one made it to the top successfully.
Ballinger described the moment he knew he had to turn around. AT "1500 feet from the summit, [it] got dangerously cold," he said. Ballinger was "shivering uncontrollably," he added.
"There was no question in the decision. I had to turn around. I had to come down," said the explorer, who has reached the summit of Everest six times before (but each time with oxygen).