To climb the remaining 800 metres to the mountain’s summit and return to camp before hypoxia - oxygen deficiency - takes its lethal toll, it is necessary to start out after 10pm. That way, the downward section, when exhaustion has set in, is completed in the relative safety of daylight. The slow-motion ascent can take up to 12 hours; just over a metre a minute. Along the way, somewhere out in the darkness, on either side of a narrow ridge, are sheer drops of between 2,500 and 3,500 metres. It’s a journey that is completed by as many as 50 people a year these days, but it is still a daunting challenge.