It is called - melodramatically, but with chilling accuracy - the Death Zone.
Once a climber ascends above 7,000 metres on any mountain, he or she is living on borrowed time.
The lack of oxygen at high altitude means a mountaineer is prone to hypoxia, a condition in which the mind and body begin to shut down.
The thin air can also bring on fatal cerebral or pulmonary oedema - water settling on the brain or flooding the lungs.
Add to this the constant threat of hypothermia in the mind-numbing cold and searing winds on top of the world's highest peaks, plus the treacherous icy conditions.
And it becomes clear why high-altitude mountaineering is so staggeringly dangerous.
This grim lesson was starkly brought home in May last year when eight people died on Mount Everest in one 24-hour period - what became the darkest day in the history of climbing the world's highest peak.