You can only get so hurt crashing a bike at zero miles per hour. That was my revelation from the first of four wipeouts at the largest one-day cyclocross-racing festival in the U.S. Short, intense, and adrenalized, cyclocross races typically run for 45 to 60 minutes around a one- or two-mile loop that deliberately zigzags on and off dirt, grass, pavement, sand, and mud, tangling up with a number of hazards, including staircases and wooden-plank barriers that force you to dismount and run, carrying the bike.

Tough Mudder for cyclists would be an apt description, if that weren't so unfair to the deep history of cyclocross, a century-old European sport combining elements of road cycling, mountain biking, obstacle racing, and – when you throw in all the food trucks, beer gardens, and costumes – a dash of Mardi Gras. Cyclocross also happens to be the fastest-growing bike-racing discipline in America, with participation more than tripling in the past seven years, according to USA Cycling. Thousands of 'cross fanatics now show up at race festivals every weekend during the fall and winter, especially in places where the cycling weather is guaranteed to suck.

Compared with your standard road race, cyclocross requires less training, there are 90 percent fewer bike snobs, and, despite the harsh weather, it's better for spectators. "It is bicycle racing and it is really hard, but it's not as scary to give it a try as, like, mountain-bike racing, where you're going to be 50 miles out in the mountains and you've got to have your shit together," says Brad Ross, the director of the Cross Crusade race series. "Or, like, road racing, where there's all that peloton etiquette, and you'll get yelled at if you do something wrong. We're all just getting muddy and having fun, and everybody's crashing and getting up and laughing about it."

Crowding elbow-to-elbow in a mad sprint at the start of the Alpenrose Dairy Cross Crusade race in Portland, Oregon, the pack bolted a couple hundred yards before slowing to a near stop around the first sharp curve. As the course narrowed, we had to roll single file down a muddy slope so steep and wet that everybody rode their brakes, slowing to an anxious crawl. One after another, we all concluded that we couldn't possibly pedal back up the muddy hill on the far side. So we dismounted in the slop, shouldered our bikes, and ran slip-sliding upward in our mountain-biking shoes. By the time I got to the top, jumping back on my bike, the pack was strung out over so much of the course that my personal race shrank to a few riders behind and in front of me.

I followed one of these guys so closely, fixating on his back wheel, that I failed to notice an approaching muddy slope until I was halfway up. With my feet still clipped into the pedals and not nearly enough speed to get up the hill, I stood to crank hard but felt my rear tire slip. Forward motion stopped, sideways motion began, and I landed flat on my back in the soft, churned grass with a bike on top of me, nothing hurt but my pride.Cyclocross got started in the early 1900s among European pro bike racers looking for some off-season competition – racing each other, village to village, through forest and pasture. Carrying bikes over streams and fences started as a necessity but was central to the sport's identity by 1950, when the first cyclocross world championships took place in Paris. Belgium currently reigns as the spiritual home of cyclocross, but the United States is coming on strong. Permanent cyclocross parks have been built in California, Colorado, and two Kentucky cities, including Louisville, where the world championships were held in 2013, the first time that event occurred outside Europe. Americans didn't exactly dominate – no man placed higher than 19th – but American sales of 'cross bikes have spiked so dramatically that the major manufacturers are radically expanding their offerings. California-based Specialized, to cite one example, went from carrying three cyclocross models in 2011 – sturdy road bikes, in essence, with narrow knobby tires – to 15 for the current product year. In a beginner's race, of course, any old mountain bike will do – or you can just slap knobby tires onto your road bike.

Part of the appeal, as I confirmed in my second crash of the race – speeding too fast into a muddy straightaway, losing my line, getting face-to-face with a grassy berm – is that cyclocross bears a high risk of bruising and scraping and almost no risk of death or maiming. "The fastest anybody is going here is 15 miles an hour, even these elite guys," said Brad Ross, when I stumbled up to his trailer after my race. "It's not like you're going 50 miles an hour in a road race descent and if you crash, a helicopter is coming." Pointing to a nearby white tent, he said, "We have two medics and they've been banging it all day, and the worst injury we've had is a dislocated kneecap."

It also helps that you don't have to live near great mountain-biking trails, or burn whole Saturdays on epic road rides down highways choked with cars and trucks. Cyclocross races are so short that you can get fit enough for the mellower heats through spin classes or just cycling to work – carrying your bike over curbs and up your office building's stairwell, maybe ripping through some dumpy local park on the way home, tearing around the baseball infield. Plenty of guys train ferociously hard for cyclocross, but it almost doesn't matter once you're in a race: The grunting of guys behind and in front of you combines with the constant mental stimulus of turning, stopping, starting, and dismounting to create a roller-coaster level of engagement and excitement, regardless of how fit you are.

"It's frantic and urgent from the minute that whistle blows," says Sean Estes, a Specialized regional team rider who competed at Louisville. "It's a scramble-mob mentality, almost a fight-or-flight thing, and you know every second counts and you just go into attack mode." Elite riders, as a result, tend to combine the horsepower of track-cycling sprinters with the technical abilities of BMX and mountain-bike racers.

Cyclocross has its own unique skill set, learned by trial and error in the race or from outfits like CX Nation of San Francisco. It offers cheap courses on how to dismount and remount without losing momentum and how to carry the bike. Putting a bike on your shoulder takes some getting used to, but according to USA Cycling, the committee that oversees official cyclocross events, one lap on the course has to be "at least 90 percent rideable," meaning you won't be forced to spend more than 10 percent of the time on foot. These courses also offer a slew of tricks for handling the terrain. CX Nation's Brian Staby, for example, teaches riders how to approach hairpin turns wide and then go tight around the inside before exiting wide again. He also counsels looking for the least slippery line on muddy and slick straightaways, especially if they slope to one side – advice I failed to take during my third crash that day at Alpenrose, when I ripped through a down-slope course boundary tape and tumbled ass-over-ankles onto a wet lawn.Course designers deliberately lay out races in cloverleaf patterns, with constant out-and-back mini loops, making it remarkably spectator-friendly. By standing almost anywhere along the course, you can see an awful lot of action. Plus, with so many events back-to-back all day long, hundreds of riders are always either done for the day or waiting for their own ride, leaving them free to join friends and families crowding the sidelines, drinking, eating, clanging cowbells, and engaging in so-called "hand-ups," reaching out to passing racers with open beers and even rolled-up dollar bills taped to the breasts of inflated nudie dolls. Racers and onlookers alike wear ridiculous costumes at many a cyclocross race. Bend, Oregon, hosts a fully costumed Halloween 'cross race on the grounds of the Deschutes Brewery, and San Francisco has at least one underground cross-dressing race series.

Then there's the joyous heckling, the universal cyclocross tradition of onlookers screaming at every passing racer. I was the recipient of this after crash number four, when I got cocky and maybe a little sloppy, flying into a down-and-back-up-again sideways-canted curve so fast that both wheels slipped out at the same time, sending me face-first into the mud. Before I could feel sorry for myself, some maniac with a half-eaten bratwurst leaned over, hollering at me to get back up. I surprised myself by doing just that, wrenching my feet out of mud-caked pedals, peeling myself off the hillside, and forcing my handlebars back into alignment over my front wheel. Jumping back on the bike again, while that total stranger shouted, "Go, dude! Go!" I sprinted onward and crossed the finish line near the back of the pack, thrilled, rattled, and without a single injury.