This past Saturday, three people died and two others disappeared while descending from the summit of Mount Everest. But it wasn’t an avalanche or a snow-storm or a deadly fall that led to the tragedy. Instead, they had simply started climbing too late in the day, due to a traffic jam of climbers trying to get to the summit during a short window of good weather. As a result, they ran out of supplemental oxygen before they could safely return to base camp. They died of altitude sickness and exhaustion, mere hours after standing at the top of the world.

Ed Viesturs has climbed Mount Everest seven times, including one summer in 1996, when 11 fellow climbers perished on the mountain. He is also the author of "No Shortcuts to the Top: Climbing The World’s 14 Highest Peaks," and he lays out the combination of factors – both physical and psychological – that all too often lead to tragedy.