Each year about four dozen athletes gather in Minnesota for the St. Croix 40 Winter Ultra. Runners spend good money to embark on a 40-mile ultra-marathon, at night, in January, in Minnesota, while pulling a sled packed with 30-plus pounds of supplies. In this environment, you can literally die from standing still for too long.

Over 25% of the runners will not finish the race. Most of these will drop out at a very interesting point. Participants reach mile marker 24 (aka Checkpoint 24) between 10 pm and midnight. If a runner plans to take on the last 16 miles, he/she must prove they have the skills to stay alive in the case of an emergency. They must stop, set up their bivy sack (a body-shaped tent that envelops their sleeping bag), climb into the makeshift bed, wait around 30 seconds, then pack it all up before leaving.

Personally, that sounds like the easiest part of the race. But when the temperature nears zero, and you're covered in sweat, coming out of a very brief respite in a sleeping bag the temptation to quit is strong. The most dangerous thing a runner can do in a race like this is stop.

Source:

Sarah Scoles, “Hell? Yes; Endurance athletes and the pleasure of pushing it,” Popular Science (Summer 2020), pp. 38-45