AP Photo/Brett Butterstein
An unidentified runner, left, and his pacer run on Engineer Pass during the sunset near Ouray, Colo., Friday, July 8, 2005, during the first day of the Hardrock 100 ultramarathon run.
Marathons, triathlons, and adventure races are extreme tests that force participants to push themselves in new ways physically and mentally.
But the most extreme endurance races in the world take that to another level.
These competitions test the absolute limits of the human body and mind.
In some cases, participants subject themselves to crossing more than 100 miles of desert, mountains, or ocean. In others, racers plan to go days with almost no sleep, reaching the point of hallucination and breakdown. There are even some races in which participants don't even know when the race will start or what sorts of obstacles they'll have to endure.
But they know it'll be brutal.
These are the most extreme endurance competitions in the world. Finish one of these and you can say you've truly challenged the limits of what's possible.