It is minus 25 degrees C, but with the chill in the frigid north wind, it is probably nearer minus 35 degrees C, twice as cold as the inside of a domestic freezer. The sun has already made its very brief appearance for the day, and has disappeared behind a mountain. This is a warm day in northeast Greenland in winter, and it’s time for a swim. After a week’s intensive survival training, seven young men are about to be tested ‘beyond the limits of their endurance’ in the Danish Special Forces SIRIUS patrol’s survival week. They will be deprived of food, shelter, sleeping bags, warm clothing, and pushed to the limit. Survival week simulates all of the worst things that can happen to anyone in this hostile wilderness, barring a polar bear attack, and that adds up to a lot of very bad things. The seven will get to know hunger, thirst, extreme cold, exhaustion and disorientation, probably for the first time in their lives, but the idea is that the experience will both psychologically strengthen them and prepare them for a very unusual job. The chosen five who make it will join an elite squad of the Royal Danish Navy, comprising 12 men and 90 Greenlandic sled dogs, who patrol a wilderness area 10,000 square kilometres in size for 26 months at a stretch, with only a one week break for rest and recovery.