Tech: 9 of the most extreme endurance challenges in the world

The most extreme endurance races in the world test the absolute limits of the human body and mind.

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.

The Brutal might be the hardest triathlon in the world.

Picking a single "hardest" triathlon is almost impossible, as each present unique challenges. Competitors need to be able to swim, bike, and run under extreme conditions.

But the double-Ironman-distance Brutal is up there. Located in Wales, it involves a 4.8-mile swim, then a 224-mile bike ride, followed by a 52-mile run.

If that doesn't sound intense enough, the "triple" Ironman version is rumored to return in 2019.

"If your dreams don't scare you, they're not big enough," the race website says.

Only 15 people have finished the Barkley Marathons race since it began in 1986.

The Barkley Marathons has no website — the way to enter the ultramarathon is a secret.

The race begins an hour after a conch shell is blown by founder Gary "Lazarus Lake" Cantrell — which can happen anytime between midnight and noon on race day. The course is said to snake 100 miles through brutal terrain in the mountains in Frozen Head State Park in Tennessee (though some say it's closer to 130 miles long). Racers have 60 hours to finish.

The course is unknown to racers until the day before the race and is mostly off any sort of trail — many participants get lost for hours.

Up to 40 people are allowed to compete every year, though more than 200 might apply. The pool includes winners of some of the other toughest ultra-races in the world.

Yet only 15 people have ever finished the race within the time limit.

Impressively, course-record-holder Brett Maune has completed it twice (his record time is 52 hours, 3 minutes, and 8 seconds). And a racer named Jared Campbell has finished the course three times.

Fun fact: If you’ve completed the course before and want to try again, your entry fee is just a pack of Camel cigarettes.

No one who has done the Patagonia Expedition Race can tell future competitors what to expect.

Every Patagonia Expedition Race follows a unique route. Racers form teams of four and cross glaciers, rivers, mountains, forests, and plains.

They might kayak, mountain bike, or rock climb, potentially traveling hundreds of miles over multiple days.

"Why are we here time and again? What invisible force attracts all of us here?," Race Director Stjepan Pavicic asked during the 2016 closing ceremony held in Puerto Natales, Chile.

The answer: "The place, the challenge, and the human endurance experience … sometimes we need to return to our original home in the wild, to reconnect with where we’ve come from."

In the Marathon des Sables, competitors brave sand and sun to race more than 150 miles through the Sahara Desert.

If you've ever thought of crossing the Sahara Desert on foot, you're not alone.

Temperatures there top 120 degrees Farenheit, and the sand and wind are powerful. Yet every year, people manage to complete the course, which is arguably the toughest footrace on Earth.

You can read Business Insider deputy executive editor Matt Turner's account of what it's like here.

In the Hardrock 100, ultra-runners take on the mountains.

In some races you're competing against your fellow racers, while in others you're mostly challenging yourself. For Hardrock, runners take on the mountains.

The 100-mile run in the San Juan mountains of Colorado involves 33,050 feet of climb and 33,050 feet of descent, with an average elevation of 11,186 feet.

Racers get 48 hours to finish the course. In 2014, record-holder Kilian Jornet finished in under 23 hours.

If running doesn't fun, you could always try the Vendée Globe — a solo sailing race around the world.

Competitors in the Vendée Globe sail around the world alone, without stopping or receiving any help.

Sailors depart and return to Les Sables d’Olonne on the French coast. They sail down the North then South Atlantic, go around Antarctica, cross the Indian and Pacific Oceans, then come back up the Atlantic.

The record finish time is held by François Gabart, who returned to port after 78 days.

Cyclists competing the Race Across America sleep so little that many say they've hallucinated while riding.

The 3,000-mile bike trip starts in Oceanside, California and ends in Annapolis, Maryland, crossing 12 states. It includes 175,000 feet of climbing.

The race has to be finished in 12 days, which leaves riders little time to sleep. The Race Across America website says lead racers usually sleep an average of 90 minutes per day — which means that by the end, many have endured sleep deprivation combined with extreme physical exertion. Those can combine to cause some intense hallucinations.

Last year's solo-riding winner finished in eight days, nine hours, and 34 minutes.

No one knows what they're getting into when preparing for the Death Race in the Green Mountains of Vermont, since no two races are alike.

The obstacles in the Death Race, both physical and mental, are unknown before the start. The race can last more than 70 hours and require participants to chop wood for hours or trudge around with a pack full of rocks. Some have even had to complete memorization challenges after climbing up a hill under barbed wire and jumping into an icy pool.

The organizers write: "We provide no support. We don’t tell you when it starts. We don’t tell you when it ends. We don’t tell you what it will entail. We want you to fail and encourage you to quit at any time."

In the Self-Transcendence 3,100 Mile Race, racers must complete 5,649 laps around one block in Queens, New York within 52 days.

The Self-Transcendence 3,100 Mile Race is a true demonstration of the fact that the most extreme endurance feats in the world are mental ones.

The course doesn't offer ever-changing natural vistas. Instead, racers must complete about two marathons a day circling the same block in Queens.

The race is put on by followers of Sri Chinmoy, a spiritual leader who taught that athletics could be a form of meditation. If ultrarunning a test against the self, this race that puts that challenge front and center.

"The serious athlete must have tremendous courage, physical stamina, concentration, and the capacity to endure fatigue, boredom, and minor injuries," the organizers write.

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