It’s called the Barkley Marathons and it’s held every year in late March or early April in Frozen Head State Park near Wartburg, Tennessee. The race started in 1986 and currently consists of a 20-mile loop with no aid stations except for 2 points of water, and whatever the runner has in their car when they go around the loop.
Runners are supposed to go around the loop five times, and they are supposed to alternate direction every time they complete a loop. It’s considered the toughest ultramarathon in the world because you also have to climb over 54,200 feet over those 100 miles.
Since 1986, about 800 people have tried this race, and only 14 have completed it. On top of being really tough, there’s also a time limit. You only have 12 hours to complete each loop. Most people finish in over 50 hours. That’s over 2 days of straight running!
The race is crazy, and the organizer is crazy too. For example, there’s no registration website, and the registration dates, and even the dates of the race, are never publicized. Check out the source to read more about it.