Mountain climbers ascending the peak of Mt. Everest (29,029 feet; 8,848 meters) will find the air pressure to be about a third of what they are used to. That’s important not so much because of the pressure itself, but because it means there is also two-thirds less oxygen to breathe. When climbers reach 26,246 feet (8,000 meters) they are at the point above which is the “Death Zone.” Many people carry oxygen tanks with them when they reach this height, although some consider it a particular test of their mettle to go without. “High altitude sickness”—hypoxia—causes fatigue, distorted vision, confusion, and loss of memory and appetite. Cerebral and pulmonary edema will set in if a person goes too long before returning to normal air pressure, and this can prove fatal within a couple of days. Frigid temperatures and lack of oxygen have killed dozens of people who have attempted to reach the top of the world. Overall, more than 150 climbers have died on the world’s tallest mountain.