If you put Everest into the Challenger Deep, it’s summit would be more than a mile below sea level. On the other hand, seven miles (the distance we’ll go down) is just half the length of Manhattan island. What makes walking the length of Manhattan, summiting Everest and reaching the bottom of the Challenger deep such wildly different challenges is air pressure. Pressure change along the length of Manhattan is essentially zero. From sea level to Everest? About 70% less air pressure at the summit than at sea level (1013 millibars to 253). Pressure at the bottom of the Challenger Deep? Over 1,000 times that at sea level (15,750 pounds per square inch, versus 14.7 at sea level).