1. Race Across America, USA
From Oceanside, California to Annapolis, Maryland, the Race Across America (RAAM) is a 3,000-mile coast-to-coast challenge that's probably the world's toughest single-stage bike race. Unlike the multi-staged professional tours of Europe, the RAAM is an extended time trial open to both professionals and amateurs alike. The current race route takes riders across 12 states and involves approximately 170,000ft of vertical ascent. Competitors can take part in teams or go solo, and have a maximum of 12 days to complete the course, averaging 250 miles per day. Each year, half the competitors drop out of the race due to exhaustion or other complications.
Fact: Austrian Christoph Strasser holds the record time, covering 3,020 miles in seven days, 15 hours and 56 minutes in 2014. He slept for fewer than 10 hours in that period, using hypnosis to minimise the effects of sleep deprivation.
2. Marathon de Sables, Morocco
Dubbed by the Discovery Channel as the most difficult footrace in the world, the Marathon de Sables is a masochist's paradise involving 156 miles of running across hot sand. First held in 1986, the ultra-marathon takes place in the Moroccan Sahara and requires racers to be self-sufficient (they must carry all food, water and supplies) to survive the six-day event. Coupling this with temperatures that frequently surpass 40C, the Marathon de Sables is as much a test of mental strength as it is of physical endurance.
Fact: Following a particularly bad sandstorm in the 1994 race, former Olympian pentathlete Mauro Prosperi was lost in the desert for 10 days. He was eventually found in Algeria, 186 miles off route and 18kg lighter. He survived by drinking his own urine and the blood of bats before being rescued by the Algerian military.
3. Ramsay's Round, Scotland
On 9 July 1978, just before noon, a semi-delirious Charlie Ramsay was spotted running down the slopes of Ben Nevis, a look of determination in his eyes. As he walked into Glen Nevis Youth Hostel, he became the first man to summit the 24 Munros near Fort William in fewer than 24 hours. The route, devised by Ramsay himself, has become one of Britain's classic endurance fell running challenges. Over a distance of 56 miles and involving 28,500ft of climbing (about the height of Everest), Ramsay's Round remains one of the toughest physical challenges in Britain, with only 84 finishers as of 2015.
Fact: Nicky Spinks finished the Ramsay Round in 2014 with a time of 19 hours and 39 minutes, becoming the only person to complete all three 24-hour British mountain courses in fewer than 20 hours. She currently holds the fourth fastest overall time for Ramsay's Round.
4. Iditarod Trail, Alaska
A trial of man and beast, the Iditarod Trail is an annual sled dog race that crosses 1,000 miles of Alaskan wilderness through blizzards and whiteouts at temperatures of -50C. “The Last Great Race on Earth” follows the route of the Great 1925 Serum Run to Nome. As diphtheria threatened the Alaskan natives, 20 brave mushers and 150 dogs delivered lifesaving medicine 674 miles from Nenana to Nome. The modern race was first held in 1973 in memory of the event and now more than 50 mushers enter each year.
Fact: The 1978 race, despite lasting over 14 days, was won by a nose (literally). Dick Mackey's lead dog crossed the finish line one second before reigning champion Rick Swenson's dog, even though Swenson himself crossed the line first. The race was awarded to Mackey but Swenson went on to be the event's all-time greatest competitor, completing 34 races and winning on five occasions.
5. Jungle Marathon, Brazil
With anacondas in the ponds, piranhas in the rivers and jaguars in the trees, Brazil's Jungle Marathon is not for the faint of heart. The most dangerous part of the 158-mile course isn't the animals, however, but the climate. At nearly 40C and 99% humidity, the risk of overheating is high — runners need to constantly gauge a sweaty medium between speed and safety. The course traverses swamps, river crossings, steep descents and narrow trails, fluvial beaches and harrowing climbs. Roughly 25% of entrants drop out before reaching the end.
Fact: The winner of the 2015 race, Thomas Wittek of Germany, was stung by a stingray before the race even started. In spite of the injury, he went on to win in 40 hours and 10 minutes, beating Andres Lledo in second place by half an hour.