
Climbers regard 8000 metres and up as "The Death Zone".  Even on 100% Oxygen,
you are slowly dying.  At 8848m (Everest), most climbers spend only a short
period of time before descending.  I've been above 8000 once.  Descending as
little as 300m feels like walking into a jungle, the air is so thick.  Everest
in winter without oxygen, no support party (Alpine style).  That is the
"ultimate challenge" (or is it solo?)