Mountains above a certain height constitute another division of the world, a separate realm of sky. The altitude above which we are required to wear oxygen masks if the cabin pressure fails is 10,000 feet, and so this rough contour shaped by peaks and an added safety margin forms perhaps the map of the world that a pilot might draw most easily from memory, as if sea level had risen by about 2 miles.
This actually doesn't quite make sense. What does the 10k oxygen mask rule have to do with this? These are where you can't drop altitude in case of loss of pressure to avoid using masks because of high mountains?
