WARTBURG, Tenn. — On Friday night, in the Cumberland Mountains of eastern Tennessee, 28 men and 7 women will lie in tents half asleep in anticipation of hearing a conch shell being blown at Big Cove Campground in Frozen Head State Park. When they hear the call, which will arrive sometime between 11 p.m. that night and 11 a.m. Saturday, they will know they are 60 minutes from the start of an ordeal once referred to as a “satanic running adventure.”

It is a 100-mile footrace that some say is actually 130 miles or more, through unmarked trails that have names like Meth Lab Hill, Bad Thing and Leonard’s Buttslide and that are choked with prickly saw briers. Temperatures often range from freezing to blistering on the same day, and there is a cumulative elevation gain of more than 60,000 feet, or the equivalent of climbing Mount Everest twice from sea level.

A 60-hour time limit forces competitors to run, climb and bushwhack for three days with little or no sleep. They endure taunts from the race director, who deliberately keeps the competition’s entry procedure a mystery. It is a race in which there are no comfort stations, and runners cannot use a GPS device or a cellphone.

Less than 2 percent of the nearly 800 ultrarunners who have subjected themselves to this punishment — 12 men, the same number as have walked on the moon — have finished the race in its current iteration. The only prize is that after 100 miles, they get to stop.

This is the Barkley Marathons, the world’s toughest and most secretive trail race.

“The Barkley is a problem,” Gary Cantrell, 59, the race’s director and creator, said recently. “All the other big races are set up for you to succeed. The Barkley is set up for you to fail.”

As ultrarunning has increased in popularity, many of its signature races have evolved from low-key affairs to big-time events with corporate sponsors and entry fees of $1,000 or more. The Barkley costs just $1.60 to enter and has not grown because Tennessee park officials will not allow more than 35 runners a year. But with the sport’s popularity on the rise and the Barkley about to be featured in a documentary, many connected to it hope the competition known as the Race That Eats Its Young can maintain its eccentric, counterculture charm.

Cantrell got the idea to create the Barkley in 1985 after learning that James Earl Ray, the man who assassinated Martin Luther King Jr., had managed to cover only eight miles in 54 hours after escaping from Brushy Mountain State Penitentiary. The prison, closed in 2009, is now part of the Barkley course. Runners wade into a stream that passes through a tunnel under the prison, and come out the other side, near the wall where Ray escaped.

The derelict prison sits on the eastern edge of Frozen Head State Park, where Cantrell liked to hike with his friend Karl Henn, known as Raw Dog. Cantrell and Henn, who became a race co-director, thought they could fare better than Ray, who was found lying face down in a pile of leaves, cold, hungry, scratched to pieces and utterly defeated by the terrain.

The next year, the Barkley, which Cantrell named after a friend and longtime supporter, Barry Barkley, was held for the first time. The course covered 50 miles, and there was a 24-hour time limit. None of the 13 runners came close to finishing.

“A rousing success all around,” Cantrell said of that first race in a story for UltraRunning magazine.

Ed Furtaw, a man known as Frozen Ed, became the first person to finish what was then a 55-mile race in 1988. The next year, Cantrell decided to make the Barkley more difficult by creating a 100-mile race, currently consisting of five 20-mile loops, while retaining a 60-mile Fun Run. Mark Williams, a Briton, became the first person to complete the 100-mile race, finishing in 59 hours 28 minutes in 1995. Only 11 others have completed the 100-mile race since. (There was no race in 2002.)

Don’t Apply Early or Often

Figuring out how to enter the race is an achievement in itself.

“There is no Web site, and I don’t publish the race date or explain how to enter,” said Cantrell, an accomplished ultrarunner who has never come close to completing his own race. “Anything that makes it more mentally stressful for the runners is good.”

(The start of the race involves a curious tradition in which participants try to refrain from letting Cantrell see them run. They will walk the first few hundred meters, until they turn a bend and begin running once they are out of his sight.)

Because so few participants are allowed, the details of how to apply are a closely guarded secret. The first step is to figure out where and when to send a required essay on why one should be allowed to compete.

“If you send it in five minutes early, he’ll delete it,” said Beverly Abbs, a 48-year-old environmental scientist from Red Bluff, Calif., who completed three loops in last year’s race. “We had to send the application at midnight on Christmas Day in Gary’s time zone, and you have to figure out which one it is on your own.”

Abbs submitted a poem reminiscent of “ ’Twas the Night Before Christmas” as her essay at 12:01 a.m. She feared her application had been denied after her husband, Alan, received, but she did not, the traditional “condolences letter” that warns accepted runners that they have a “very bad thing waiting.” It was signed by Lazarus Lake, Cantrell’s nom de guerre. A day and a half later, she received the letter and realized that the delay was an attempt by Cantrell to inflict “psychological torture” on her.

This year’s application said that women were “too soft” to conquer the Barkley, but Abbs, who has completed 11 other 100-mile races, said she was confident in her ability to prove him wrong.

“I will either be pulled off the course, or I will finish the race,” she said.

The field is made up of those who have completed the race, athletes with impressive ultrarunning credentials and veterans like Leonard Martin, a dentist from Knoxville known as Buttslide. He has failed to finish in 17 attempts. Others are plucked randomly. And Cantrell selects at least one “sacrificial virgin” whom he believes has no chance of completing the race. (This year, it is Ryan Brazell of Rhode Island.)

Runners are also required to complete a bizarre entry form with questions like, “What is the most important vegetable group?” In addition to the $1.60 entry fee, first-time entrants must bring a license plate for Cantrell’s collection, which he displays next to the race’s starting gate. Veterans who have never finished are required to bring him a specific article of clothing — this year it is a size 18 flannel shirt — while those who have finished need only bring a pack of Camel cigarettes, which Cantrell calls his “retirement plan.”

Only after they have completed the entry form are runners given the date of the race.

With a total elevation change of 121,560 feet this year, there are only a few flat sections on the course. To confirm that no one is cheating, Cantrell hides 10 books at various points, often ones with titles appropriate to a given section of the course: “Death Walks the Woods,” “Heart of Darkness,” “A Time to Die.”

Runners have to rip out the page that matches their race number. If the page is lost, the runner is disqualified. Last year, 22 of 35 runners gave up on or after the first loop. Everyone who fails to finish experiences the indignity of listening to Cantrell play “Taps” on his bugle in a gleeful homage to the triumph of nature over man.

“ ‘Taps’ rings out all night long on the first night,” said Cantrell, who sits by the starting gate with his dogs, Little and Big, smoking Camels and drinking cans of soda, often staying up all night.

They Went That Way

Six hours before the race starts, Cantrell posts a course map that runners must copy onto maps they purchase, and he gives runners a printed set of directions, which can often be hard to follow, even with a compass. (Runners are split up if they make it to the final loop, with alternating racers running in opposite directions.)

Dan Baglione, a retired computer scientist and experienced ultrarunner, set a record for futility in 2006 at age 75 when he became hopelessly lost, eventually wandering into a different county on a 32-hour odyssey in which he was credited with covering just two miles of the actual course.

“There is no trail,” said Baglione, who insisted he would try the Barkley again if he had not been barred from returning. “And the instructions are hopeless.”

A documentary film crew was recording last year’s race when a camera operator became lost for 16 hours before a search party was dispatched to find him.

“That experience endeared them to everyone,” Cantrell said.

Brett Maune, a 34-year-old physicist from the Bay Area, finished the Barkley in 2011, then beat the course record by nearly four hours last year, when the Barkley had an unprecedented three finishers. No other runner has finished the Barkley twice.

Brian Robinson, a 51-year-old from California who became the first person to hike the Appalachian, Pacific Crest and Continental Divide trails in a calendar year, in 2001, failed in his first two attempts at the Barkley, in 2006 and 2007, years when no one finished. But he completed it in 2008.

“I wanted to test my limits,” Robinson said when asked to explain why he kept returning. “The Barkley is good for that because pretty much no one can finish it.”

Cantrell said most Barkley finishers had a background in science or engineering and all but one had an advanced degree.

Sleep-deprived runners who make it deep into the race tend to hallucinate, and the few who make it to the finish line are shattered when they arrive. Maune said he was incoherent after his first finish.

John Fegyveresi, a 35-year-old Ph.D. candidate in geoscience at Penn State who finished the race just minutes before the deadline last year, was told that he devoured an entire pint of Ben & Jerry’s chocolate chip cookie dough ice cream at the finish line. He has no recollection of that.

“I couldn’t walk the next day, so I stayed in the campground for a day and a half,” he said. “Then I tried to drive home, but after 20 miles I was falling asleep, so I had to check into a hotel. I slept for 16 hours and then missed the checkout time.”

Fegyveresi has also completed the Badwater in California, a 135-mile Death Valley marathon that bills itself as the world’s toughest footrace. He said the Barkley was considerably harder.

“It was 120 degrees at Badwater, but it’s a flat road and you have a crew following you, so you don’t have to carry anything and you can’t get lost,” said Fegyveresi, who needed 21 fewer hours to finish Badwater.

Cantrell claims, somewhat improbably, that he wants runners to finish his race.

“Humans are made to endure physical challenges,” he said. “The real joy is seeing people who find something in themselves that they didn’t know was there.”

Few would dispute the difficulty of the race, but some renowned ultrarunners avoid it, saying that because it involves navigation and an unmarked trail, it’s more of an adventure race than a true ultramarathon. Others, like Charlie Engle, who once ran 4,500 miles across the Sahara, have tried but failed to complete even the Fun Run portion.

Running in Silence

The top performers at the Barkley are far from household names and seem content to keep it that way.

“I hope the race remains more of a secret,” said Maune, who will not compete this year because of a back injury, when asked if he hoped the running world would recognize his Barkley achievements.

Fegyveresi said that older Barkley veterans wanted the race to remain obscure and were worried about the effect of the documentary, but some of the younger athletes want the race to receive more recognition.

When this reporter announced his intention on the Barkley e-mail group list to write about this year’s race, some race veterans did not seem pleased.

“Please don’t send us spectators, troublemakers or yuppies,” wrote one.

“Don’t write about us. There is already too much information out there,” another said.

Few would consider divulging the race date, and even among themselves, many runners, including Fegyveresi, would not confirm if they were competing this year.

“You never know who’s going to be there until you show up,” said Martin, who was at Frozen Head training for this year’s race recently. Cantrell insisted that as long as he was alive, the race would stay as it is. But he too faces pressures. Two years ago, he lost his job as the treasurer of Shelbyville, Tenn., and has been out of work since.

He has kept the race’s original entry fee, which represents a penny a mile for the 60-mile Fun Run and the 100-mile race.

Cantrell said he had declined offers of up to $1,000 from those who want to get into the race, and maintains that the Barkley means too much to him and to others to sell entries.

“You can’t buy the Barkley,” he said.