Climbing Mount Everest without supplemental oxygen is a feat only about 200 people in the world have ever completed; prior to 1978, when Reinhold Messner and Peter Habeler made the first such ascent, it was considered impossible. In the thin atmosphere of Everest’s peak, you breathe in less than a third of the oxygen you’d take in at sea level—and a lack of O2 not only makes exercising incredibly more taxing, but takes a toll on your thinking capacities, too.