A strong contender would have to be the Marathon des Sables, around 250 kilometers (156 miles) worth of racing through the Sahara desert in southern Morocco. C

A strong contender would have to be the Marathon des Sables, around 250 kilometers (156 miles) worth of racing through the Sahara desert in southern Morocco. Contestants get access to rationed water and a tent. All food has to be carried by the runners. Along the way, they get to deal with dehydration, blistering heat, sand storms, and the very real risk of getting lost. One stage is typically about two marathons in length, and has a time limit. It's one of very few events that require insurance for corpse repatriation. Up to 2008, two deaths occurred, one of whom died of a heart attack in his sleep after completing the long stage in 45th place (out of about 750). To gain entry, you have to have a medical certificate anda resting ECG report from at most 30 days before the start of the race, so that's quite extraordinary.