Mountaineering term for the territory above 25,000 feet (about 7600m), due to the hostile conditions there. The name is well-earned: for example, as of May 2001, the number of people who died while climbing Mount Everest was 168, compared to about 1300 who successfully got to the top and back again.

The main reasons for this:

  1. Extreme cold, aggravated by strong winds - this is what actually kills people (well, except for avalanches and falling, which are just as much a threat as in lower regions).
  2. Rapidly and sometimes unpredictably changing weather conditions.
  3. Long distances back to the base camp.
  4. Overconfidence, ambition and group dynamics.
  5. Low oxygen levels causing altitude sickness, which in turn causes a) weakness and b) impairs judgement.
  6. Impossibility of external help
Because of point 1 above, you can basically only survive up there when the weather is good. If it turns bad and you don't get back to the base camp in time, you die. So the general idea is to wait in the camp until there are stable, good conditions, then go up to the summit and back to the safety of the camp as quickly as possible. But due to point 2, the "stable" conditions often aren't, due to point 3, you have to turn back at the first signs of a weather change, which some people due to points 4 and 5b don't, or which turns out not to have been early enough due to point 5a. The book Into Thin Air documents a particulary bad case.