For those who haven’t summited 29,029-foot Mount Everest, it’s nearly impossible to imagine the immense physical and psychological challenge of climbing to the top of the world. I’ve been as far as Base Camp, at 17,600 feet, where there’s about 50 percent of the oxygen in the air as there is at sea level. Even at that elevation, I felt weak and wholly unlike myself. As you get closer to the summit, the terrain is far more precarious and oxygen drops to 33 percent of sea level. “It’s like climbing stairs and holding two out of every three breaths,” says Alan Arnette, who summited in 2011 and covers Everest news for Outside. “And that’s while using bottled oxygen.”
In an attempt to bring the madness of the Death Zone into focus, here’s what’s going on in your body while climbing Everest.