Last year, more than 350,000 people ran through a gauntlet of obstacles in Spartan Races across the country, competing for a chance at prize money and bragging rights. Since the first race in 2010, which drew close to 500 runners to Vermont, the event has grown exponentially; so far this year, more than 60 races are scheduled in the U.S., U.K., and Canada – 30 this summer alone. Devotees are so passionate that some locations are chosen by crowd rules: Last year, nearly 4,000 people from the Gulf Coast barraged the company with calls and emails demanding an event near them (shown here). "Finally, I threw down the phone and said, 'Screw it, I'm going down to Mississippi to see what the hell is going on,'" says founder Joe De Sena, a 43-year-old ex-Wall Streeter and rabid endurance athlete (he once did two ultramarathons and an Ironman in one week). De Sena created the races – a three- to five-mile "Sprint," eight-mile "Super," or 12-mile "Beast," loaded with obstacles like mud pits, walls of fire, and barbed wire – to "get people off the couch" and test their limits. Asked why he thinks so many are willing to literally drag themselves through the mud, he says, "I think human beings are meant to climb, crawl, jump, swim. The past 150 years, everything we've done has been to make things simpler, cleaner, more organized. And although it feels very natural to our society, those are very unnatural things. I see people out in the woods, and they feel great."

More Information: All of the Spartan Race events are open registration, except the Ultra Beast, which requires an application. [spartanrace.com]