Due to Denali's high latitude, there is about half the oxygen in a lungful of air than you'd have at sea level. Your body needs O2 to make energy, so your ability to do work just takes a bit longer at this elevation.
Cutting blocks one time back in the '90s, we'd finished cutting our first row when the first block we started to remove fell down and out of sight! Our first thought was CREVASSE, but it turned out to be an old snow cave. That was pretty exciting.
The 2021 season saw a handful of individuals and teams attempt the summit from 14,200'. Some of those attempts resulted in rescues. The upper mountain of Denali demands respect and unless you're fully acclimatized, and super strong, please don't even consider a shot from 14,000' Camp.