The desert race is 135 miles long, in the Death Valley National Park, California. That is roughly the equivalent of just over five regular marathons, in the desert.
It starts in the infamous Death Valley, finishes at Mount Whitney - around 8000ft above sea level and is run over as much time as you need.
The men's record is 22 hours and 51 minutes, the women's 26 hours and 16 minutes, although most manage it in a 2-3 days. If they finish, that is - usually around a third of runners fail to make the final peak.
The temperatures can reach a toasty 55 degrees Celsius, with mid-July picked for the race because it boasts the harshest temperatures.
Incredibly no-one has died running this race, but the gruelling nature of it means only the most experienced endurance runners need apply.
Keith Straw, 58, feels the heat
Ben Gaetos of the Philippines, 56
Ed Ettinghausen, 50
A roadside sign warns runners of the extreme heat in Death Valley
Shannon Farar-Griefer, 52, of California is given water by her team
Chris Moon of Britain (R), 51, and Shannon Farar-Griefer, 52, of Hidden Hills
Oswaldo Lopez, 41, of Mexico is doused with water
A competitor is reflected in a car mirror