In the late 1970s, Martin Luther King Jr.’s assassin escaped from prison and managed to run a measly 8 miles in 55 hours. Gary Cantrell, a local ultrarunner, figured he could have run at least 100 in that time, and so began the Barkley Marathons, an annual race in Tennessee’s Frozen Head State Park.

Participants have 60 hours to run 100 miles of the steep, thorny, and muddy trail. In other words, the run tests endurance and pain tolerance. To prove they followed the course, runners rip a page from books scattered along the trail.

Every 20 miles, the trail loops back to Cantrell, the event’s host, and a campsite of food, sleeping bags, and beer. There, runners figure out whether to endure another 20 miles or stay. Almost no one finishes.