The Barkley Marathons is a mysterious ultramarathon held every year in a state park in eastern Tennessee.

Runners have to complete five loops of a 20-mile course that most say is really 26 miles, making the race somewhere from 100 to 130 miles in total.

The race's total elevation change means finishers do the equivalent of climbing and descending Mount Everest twice.

Since 1989, only 15 people have finished the race.

For a runner who tries to enter the Barkley Marathons, one thing is certain: They have almost no chance of finishing.

The first Barkley race was held in 1986, but the course distance was bumped up to at least 100 miles — probably 130 miles, depending on who you ask — in 1989. Since then, there have been only 15 finishers.

In 2018, a year with particularly miserable course conditions, no one finished the race. A runner named Gary Robbins came closest, completing three of the five loops of the course. In Barkley parlance, that's considered a "fun run."

If ultramarathons are about testing human limits, a race like Barkley is about confronting the point at which people fail while facing those limits — or redefining success and failure entirely. Each of the race's five 20-mile loops is really more like 26 miles, most say. And finishers experience a total of 120,000 feet of elevation change throughout the course, the equivalent of climbing and descending Mount Everest twice, according to a 2014 documentary called "The Barkley Marathons: The Race That Eats Its Young."

Robbins got close to becoming the 16th finisher of the Barkley race last year, but he arrived at the finish line six seconds too late; to be considered a finisher, a runner must complete the race in 60 hours or less. Upon arriving, Robbins realized he'd skipped 2 miles of the course anyway.

Despite the hellish challenges, something draws runners to Frozen Head State Park in Tennessee year after year to endure as much as they can and almost certainly fail.

Hundreds apply every year, including the winners of other races considered to be among the most difficult in the world, like the Hardrock 100. About 40 people receive a "condolences" letter telling them they've been accepted into the race.

Those letters usually tell racers that a "bad thing" awaits, or to get ready for an "extended period of unspeakable suffering, at the end of which you will ultimately find only failure and humiliation."

A mysterious entry process

Amelia Boone, who works as an attorney for Apple, is a three-time champion of the World's Toughest Mudder competition and a Spartan Race world champion.

When Boone got her condolences to compete in the 2018 Barkley Marathons, she was totally surprised, as she thought acceptances had already gone out.

Boone, who'd been getting back into racing after a broken femur in 2016, wanted to become the first woman to complete the Barkley.

"Of the people that tend to run it, everyone is really smart — there's a lot of scientists and a lot of really big data geeks," Boone told Business Insider. "It's all these people who really haven't failed that much, if at all, and they come to this race that has a 99% failure rate."

Amelia Boone in a different competition. Reebok

No official information about the Barkley Marathons is published or publicly distributed. There's no website, since nothing about the race is supposed to be easy — including figuring out how to enter, a process that's kept secret.

The creators of the Barkley Marathons, Gary Cantrell (who goes by "Lazarus Lake" or just "Laz") and Karl Henn (who goes by "Raw Dog"), designed it to be surrounded by secrecy.

Most of the information for prospective racers comes from previous runners' accounts, of which there are plenty online.

Matt Mahoney, an ultrarunner who has attempted the Barkley 15 times but never finished, explained on his website that if someone wants to enter, they must get a person who has run the race before to reveal which day of the year to send an application to Cantrell. First-timers like Boone need to submit some sort of essay explaining why they should be allowed to run.

According to reports, once a Barkley "virgin" gets accepted, the entry fee is $1.60 and a license plate from their home state or country.

Lake hanging one of the required entry items: a license plate from the runner's home state or country. Courtesy: "The Barkley Marathons: The Race That Eats Its Young"

Rumor has it that at least one spot each year is cruelly reserved for a "sacrificial virgin" who has even less of a chance of finishing than the rest.

People who have attempted the race before are said to return for subsequent attempts with a shirt or some socks for Cantrell as their entry fee.

A course designed to break people

Because of this intentional lack of information, Boone said, "you really don't know what it is unless you're out there."

"Out there" is how many runners describe it. Ed Furtaw, who goes by the moniker "Frozen Ed," was the first runner to finish a three-loop race in 1988, before Cantrell decided the race should be five loops. Furtaw wrote a book about the race, titled "Tales From Out There."

"Unlike other ultras in which race management and volunteers do their best to help as many runners as possible finish, Barkley is intentionally set up to minimize the number of finishers, while still trying to keep it within the limits of possibility," Furtaw wrote in 1996. "Gary keeps making the course tougher when he thinks too many runners are finishing."

The hills of Frozen Head State Park have long been renowned for their difficulty.

In 1977, James Earl Ray, the man who assassinated Martin Luther King Jr., broke out of Brushy Mountain State Penitentiary. The prison walls didn't contain him, but the surrounding terrain did. Bloodhounds found Ray hidden under leaves 54 hours later, just 8 miles from the prison.

Cantrell, who was an ultrarunner in Tennessee before the sport was as popular as it is now, explained in the documentary that he heard about Ray's story and thought he could cover 100 miles in the time Ray traveled 8 miles.

So Cantrell decided to create a race through Frozen Head State Park.

The theoretically 20-mile course loops run through terrain features that Cantrell and others have given nicknames like "Rat Jaw," "Testicle Spectacle," and "Checkmate Hill." The course changes every year. While there are some trails through the park, at least two-thirds of the course is off-trail, requiring serious scrambles and occasional climbing.

"Only at the Barkley are you as likely to experience an upper-body injury as you are a lower-body injury," Robbins wrote in a race report.

In the mountainous area, weather can also be capricious, with intense winds, hail, snow, ice, and more.

"If you're going to face a real challenge, it has to be a real challenge," Cantrell said in the documentary. "You can't accomplish anything without the possibility of failure."

No rulebook

Boone said there was a kind of unwritten rule that nobody talked about the Barkley Marathons in the lead-up to the race.

"It's kind of like: I have this cool secret, and I'm training for this cool thing, but I can't tell anybody about it," she said. "But in some ways that made it even better, because for the first time in a long time I was doing something solely for myself."

She knew, however, that she'd need to practice using a compass and a map and going up and down a lot of hills.

Jared Campbell, an ultrarunner, clings to a tree while scaling "Danger Dave's Climbing Wall," one of the many challenging ascents in the course of the 2012 Barkley Marathons. Courtesy: "The Barkley Marathons: The Race That Eats Its Young"

No GPS device of any kind is allowed on the race, nor is an altimeter — just a map, a compass, and a cheap Timex watch ticking down the 60-hour time limit. Runners have to carry their own food, water, lights, and other necessities with them. Depending on how much water they bring, they may wind up drinking from streams.

Since there's no official rulebook, nothing tells runners what they should or shouldn't do leading up to the race. But they're informed that within Frozen Head State Park, racers are allowed to train only on the trails, meaning participants can't prepare for how rough the full experience will be.

Starting with a cigarette

The day before the race, after runners arrive at the camp in Frozen Head, they get to see the one official course map that denotes that year's route.

Racers have to copy the information from that map onto their own maps, writing down elevation points and the off-trail twists, turns, and climbs. Navigating the route depends on the racers' ability to follow the directions they've marked.

Then the runners wait to hear the sound of a conch being blown, which could come anytime between midnight and noon on race day. Once runners hear the conch, they have an hour to get ready to run.

The Barkley Marathons officially begins when Cantrell lights up a cigarette.

Once he lights the smoke (returning runners who have finished the race bring Cantrell a pack of Camels as their entry fee), the runners are off.

To ensure racers hit each point on the course, the organizers stash books throughout the route. Cantrell gives every runner a number before they start each loop, and when they find a book, they have to tear out the corresponding page.

But finding the books is no easy task, especially as foggy days turn into dark nights. When Boone arrived at the third book location on her second lap, she was with a group of four people. Another runner was already there and had been searching for the book for hours.

"The five of us combed the location, and, getting frustrated, I turned to find a bit of shelter to pee," Boone wrote in her race report. "And in front of me, there was the book, shoved in the crevice."

Running together, then alone

John Fegyveresi, a Barkley "virgin," runs past the decommissioned Brushy Mountain State Prison during the 2012 Barkley Marathons. Courtesy: "The Barkley Marathons: The Race That Eats Its Young"

For the first four loops, runners work mostly together, helping each other fight the course.

The loops start and end at camp, which is the only place runners can receive aid, tape up blisters, replenish food supplies, and take a nap — if they have time. Each loop has to be finished within 13 hours and 20 minutes.

Racers are supposed to do the route clockwise for the first two laps, then counterclockwise for the second two. When loop five comes around — if anyone makes it that far — the runner in first place gets to choose which direction to run it. Any subsequent runners then have to alternate, each going the way opposite whoever came before.

On that last lap, racers are trying to beat their competitors, the course, the clock, and their own exhaustion.

The documentary shows two runners — Brett Maune, a physicist who holds the Barkley speed record, and Jared Campbell, an engineer who has also won Hardrock — finishing their fourth loop within a minute of each other in the 2012 race. They both completed it, but almost four hours apart. They're the only two people who've finished the Barkley multiple times.

That year was also the first time the race had three finishers. A Barkley virgin named John Fegyveresi also made it to the end, with less than 20 minutes before the cutoff.

Most runners, of course, don't make it that far, often battling course conditions that are nearly impossible to outlast.

The course record for the least distance covered in the slowest time is held by Dan Baglione, a retired computer scientist who got lost for 32 hours after covering just 2 miles of the course in 2006.

Finding out something about yourself

In the middle of her second loop, Boone realized her group wouldn't make it back to camp in time to start a third. She'd finished her first loop in 10 hours and 57 minutes — and bonus time gets rolled over, according to race rules — but the 26:40 mark was inevitably going to pass before they made it back.

Boone and her crew still finished the loop, even if it wouldn't count. Out of 44 starters this year, Boone was one of just 21 people who even began the course's second loop.

Lake bows his head as a crew member named Dave Henn plays "Taps," which is played on a bugle every time a runner quits. Courtesy: "The Barkley Marathons: The Race That Eats Its Young"

Contrary to how it might seem, Cantrell has said he wants people to finish the race.

"Pretty much everybody we see go out there, you really want them to succeed," he said in the documentary. "You know that most of them won't, and there is kind of maybe a dark humor to all the things that go on. Some of the failures are spectacular and really funny. But you like to see people have the opportunity to really find out that something about themselves."

For Boone, attempting Barkley was about remembering what got her into racing in the first place: the fun of it.

She said that before her broken femur (and a follow-up stress fracture), racing had started to become exclusively what adventure athletes call "type 2 fun," something that seems fun only when you look back at it, not while you're doing it.

"I was only happy afterwards if I won," she said of that time. "The process of getting through it was miserable."

But Boone said Barkley was a "culmination of a year of racing again and learning to actually enjoy myself during the race."

It was about running without being afraid to fail and having "type 1 fun," the kind you enjoy throughout an activity, not just in retrospect.

Boone and every one of her fellow 2018 racers got a "DNF," or "did not finish."

"It didn't feel like a DNF — it felt like a victory," she wrote in her report, "a hard-fought victory, defeating the self-doubt demons I wrestled with leading up to the race."

Boone said she'd consider trying again.

Those who do manage to finish get quite a prize: They don't have to go out for another loop.