Carrying a big bucket full of gravel up and down a mountain isn’t what most people would consider fun.

Yet an estimated 5 million people will willingly crawl dozens of yards in dirt and rocks under barbed wire, wade in pools of mud and sometimes endure electric shocks and tear gas this year at obstacle races — a sport that has been called the fastest growing in America, according to Obstacle Race World, a website that tracks the sport.

And entrepreneurs are looking to cash in: The number of race organizers has exploded from just 15 in 2012 to 633 this year — a more than 4,000% increase, according to MudRunGuide.com, an industry resource. Last year saw more than 1,370 events, compared with 354 in 2012.

“This has really redefined the idea of what it is to be a weekend warrior. Mom and Dad just don’t wake up on a weekend morning and go for a quick jog like before,” says Hunter McIntyre, a 25-year-old professional obstacle racer who has won first place at more than 20 events. “Our world is getting out of that little petri dish lifestyle where you go to your apartment, your car, your desk and go back. You see people running through rivers and just weird shit.”

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Electroshock therapy and tear gas. Carrying sandbags through mountain ranges. Climbing up 20 feet of rope to ring a bell, without falling into the pool of mud below. Crawling through dark, wet tunnels, only to learn there’s a dead end at the end. Some of these obstacles may sound like retribution for protesting in an undemocratic country. But to the participants who sign death waivers and pay $100 or more for entry, they’re considered sport.

“The traditional sports of endurance like triathlons or running a marathon, that’s not sexy anymore,” McIntyre says. He’s right: 4 million people took part in non-traditional races like obstacle, mud or themed runs in 2013, compared with 2.5 million marathoners and half-marathoners, according to Running USA, which is still calculating those figures for 2014.

Traditional vs. non-traditional race finishers

For some, it’s easy to see the appeal. Obstacle races can be considered an equalizer. With unpredictable courses and harsh terrain, even the fittest of racers breathe heavy. Each juncture tests a different skill: Fast runners can sprint up mountains, but it takes upper body strength to hoist sandbags into the air using a rope and pulley.

“You’re going to be covered in mud. You’re going to trip. You’re going to fall,” says Brett Stewart, who runs MudRunGuide.com. “Because everyone looks like a fool, no one’s going to laugh at anyone. The obstacles are going to force everyone to walk.”

Unlike at road races, where runners set off from the starting line at the same time, obstacle races release athletes in waves to prevent backlogs at the hurdles. So even if you’re dead last of everyone in your starting time, you’ll still be surrounded by other racers and would be hard-pressed to guess your standing. There’s the added glory of posting an action shot on social media afterward, abs unleashed, covered in dirt, swinging from the monkey bars.

At many obstacle races, getting through the challenge is all about teamwork. This 5-foot-3-inch writer completed her first obstacle race last month — a Spartan Race event — and several strangers offered a boost when she approached a 10-foot wall.

“You push together to get things done. You get some people in these groups who normally wouldn’t do any of this on their own,” says Patrick Pastrana, a 30-year-old technology consultant at Accenture who moved from the New York area to London six months ago. He’s done a handful of obstacle races since his first in 2010, and is organizing his department to complete a Tough Mudder this fall. “The satisfaction comes from completing it as a unit.”

The majority of obstacle racers range from ages 25 to 40, according to Obstacle Race World, with a 60-40 ratio of men to women. That gap may narrow as many women are drawn to females-only races, such as the Dirty Girl Mud Run, a 3-mile course, and Mudderella, a 5-7 mile event.

Barbed wire crawl Spartan Race

Many races charge upward of $100 to participate. But turning a profit off them isn’t that simple. At least 100 race companies have failed since 2012, by MudRunGuide.com’s count.

“There’s this myth that you could put on a race for $10,000 and make $100,000 and walk away,” says MudRunGuide.com’s Stewart, a consultant for dozens of race organizers. He estimates that the average large-scale event costs at least a quarter-million dollars to host, after accounting for the venue, building, marketing and staffing. At traditional races, like marathons, municipal closures account for most of the expenses.

“If you put on a marathon, or a run, or an ultrarun, or an Ironman, you don’t have to build anything,” says Joe DeSena, a former trader who worked on Wall Street for about a decade and started the brokerage firm Burlington Capital Markets. He quit in 2005 to enter the race business and, in his own words, “failed miserably.” DeSena estimates he’s burned $5 million on races that flopped, including $600,000 on the first event alone.

He founded Spartan Race, now one of the biggest obstacle run brands, operating in 20 countries, in 2010. By midway through the year, his bookkeeper told him they were going to go bust.

So he called five old Wall Street buddies that weekend and asked for $1.5 million. On Tuesday, they sent him $1 million.

“People look at this company and they think, oh my god, Spartan is a billion-dollar business,” DeSena says. “It’s only five years old. It’s only a baby. I was just in the office yesterday and we were talking about, ‘we need money.’ Running a business is harder than having kids.”

At the first Spartan race, DeSena says the company hand-timed the 750 or so racers who showed up. This year, about 1 million people will partake in Spartan races. With registration fees usually over $100, the assumption would be that the company is a money machine.

Despite the company’s growth, DeSena says profit margins are thin, after factoring in course-building costs, payroll for employees, time chips, medals and shirts for finishers and big capital expenses like trucks, trailers and wood. Spartan Race declined to say how much revenue the company draws in and the average cost to stage a race.

“Anybody who’s done a kitchen remodeling, ask them how it went,” he says. “Try doing 30 kitchens on the side of a mountain for a race, with heavy equipment and lumber, and guys running out of screws.”

And some races offer winners hefty cash prizes. Spartan gives the top elite male and female finishers at its world championship a $15,000 cash prize. McIntyre, the professional obstacle racer, wouldn’t say exactly how much he makes annually nabbing podiums. But he says rockstars who compete throughout the year can rake in six figures.

Regardless of the costs, entrepreneurs looking to pull off their own race series are entering the field in droves.

BattleFrog, the brainchild of three ex-Navy Seals, staged its first race in March 2014 in Atlanta. The courses range from 400 meters for children to an 8-kilometer run with more than 22 obstacles. At its Pittsburgh race, participants run and swim through a mine shaft while wearing hard hats and lights.

BattleFrog isn’t turning a profit yet, its co-founder and general counsel Michael McAllister says. He expects to hit that marker next year as registration picks up. At their early races, about 1,500 racers showed up. That figure has since hit 4,000 for some events.

“We believe that this is really the place where sports are headed,” he says. “The demand for participatory sports has barely been touched.”

The most well-known obstacle race series, Tough Mudder, expects more than 2 million participants worldwide this year. That’s about double the number Spartan Race, also considered one of the largest brands in the business, projects it will reach. DeSena has loftier plans.

“This is going to sound ridiculous, what I’m about to say: Let’s touch a billion people,” DeSena says. “I mean, the couch got to over a billion, so why can’t I get to a billion?”

The rope climb obstacle Spartan Race

Obstacle-inspired gyms are cropping up across the country, helping newfound warriors ditch treadmills and traditional equipment in favor of monkey bars and ropes and high walls for climbing. Nearly 200 such gyms have opened nationwide, MudRunGuide.com’s Stewart estimates, because “it’s exploding.” Their price tags are, too.

New York’s obstacle-focused Warrior Fitness Boot Camp sells an unlimited one-month class package for $500, with a slight discount for three-months at a rate of $1,350. (One way to save money: Like this writer, book your class through ClassPass. The $99 fitness membership gives you unlimited classes at hundreds of New York studios, with the caveat that you only visit each individual studio up to three times a month — including Warrior.) At Gymja Warrior in Danvers, Mass., an unlimited package costs $225 monthly.

Spartan Race plans to open its own 18,000 square-foot training facility this fall at the 1 Hotel in Miami, where “you can turn the dial and one setting will be Siberia, and one setting will be Death Valley,” DeSena says. It’ll offer weeklong packages for people to stay at the hotel and get Spartan fit, and the company plans to attract corporate retreats.

Beyond extreme fitness, the industry is eyeing another goal: the Olympics.

Obstacle racing can by nature seem un-Olympic. In the Olympic steeplechase, perhaps the closest comparison, men and women jump over hurdles and a water pit of a pre-set size. Obstacle courses are touted for throwing racers into the unexpected. How could that fit in?

“There’s rhythmic gymnastics — with the little flag, they dance around. There’s ping pong,” DeSena says. “This is closer to the Olympics than most sports.”

With Boston being considered as a possible Olympic host, there’s hope that obstacle racing could serve as the exhibition sport.

In the meantime, a group called the OCR World Championship hosted its first event last year, seeking to become the independent arbiter of obstacle races. It set qualification requirements and handed out cash prizes across 16 categories.

But as much as obstacle racing is gaining traction as a professional sport, even industry insiders have called it a free-for-all. Take the fact that it has no sanctioning or governing body. Attempts to regulate obstacle racing by creating a central group have largely fallen flat. That means after requiring participants to sign off on waivers that absolve race organizers from virtually any liability, companies have free rein to send them close to the edge by building whatever obstacles they fancy — so long as they don’t destroy their reputations by hurting too many people, or killing anyone.

When you put thousands of people on unfamiliar terrain and make them literally jump through hoops, somebody is bound to get hurt. A study by the American College of Sports Medicine found that for every 1,000 participants at a Tough Mudder or Warrior Dash race, 26 to 37 people got sick or hurt. In 2013, a 28-year-old man drowned at a Tough Mudder. Many courses are built on rural land, and the Centers for Disease Control and Prevention has warned that racers could get animal feces-laced dirt in their mouths, leading to diarrhea, vomiting, cramps and fever.

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Billy Wilson, a former member of the British infantry, organized marathons in England in the late 1970s. Recalling his days designing obstacles for the guard, he decided to add a 3-foot-6-inch hurdle to a cross-country road race.

”They said, ‘they’d never jump over that.’ I said ‘they will,’ and they did,” Wilson, 78, says. “Then I started building a new obstacle at every event.”

Wilson, who goes by the nickname “Mr. Mouse,” slowly built the first obstacle race course that took on the name Tough Guy in 1988. The event now includes more than 250 obstacles on his family fields in South Staffordshire, England, in the West Midlands. At the signature challenges in January and July, racers endure a 15-kilometer course and must complete all the obstacles — walls, mud, lake swims, walls, barbed wire and more. (He says he took on the nickname “Mouse” to show he’s a gentle leader.)

Perhaps the most menacing Tough Guy obstacle: a dark underground “torture chamber” with screaming white noise and smoke, in which people must crawl in water to avoid electrical wires and metal spikes hanging above. “We haven’t seriously hurt anybody in there, but we did nearly kill a guy, so we have to be careful,” Wilson says. Once, a 6-foot 3-inch heavy man was crawling in the chamber. He ended up face-down in the water. Marshalls kicked the water out of him, and he survived.

One man died on Tough Guy’s course in 2000, “but we didn’t kill him. He had a dodgy heart,” Wilson says.

“The great thing is that all these people are recognizing these events without knowing that we started it all,” he says. “They’ve turned it into a money-making machine.”

He has no desire to turn his race into a global enterprise, he says, and runs Tough Guy as a nonprofit that donates to charities that come help marshal the event. “I don’t want to travel any further than my horse can carry me,” he says. “I go to the bank in town, three miles away, and that’s how far he wants to walk.”

As for feeling like everyone has copied his product, “there’s an old Chinese saying that says he who smiles rather than rages is always stronger. We’ll just creep along.”

He says he created Tough Guy so ordinary people could get a taste of the extremities of life and an adrenaline jolt — something he hopes will continue to grow in popularity.

“In normal life, people live sedately. Sometimes in life, you’re going to come across a problem where you don’t know how to overcome it,” Wilson says. “By taking on these psychological challenges, you have a forewarning on how to cope with life. Now, we’ve started this and we’ve got it roving along. We want to take people to absolute extremes.”