It's a Monday evening in New York, and the after-work rush is on at gyms around the city. Mike Rocks, a 32-year-old account manager at a genetic lab, is stretching quickly before jumping into his twice-a-week kettlebell routine. But he's not at his usual gym. For several months now, Rocks has been coming to the Cliffs Climbing + Fitness, a sprawling complex in a former warehouse in Long Island City, in Queens, New York. With more than 30,000 square feet of sheer, angular walls dotted with thousands of chalk-covered holds, it's one of the largest climbing gyms in the country. The $4 million facility includes all the wall space a rock hound could want, plus a traditional exercise area that's stocked with free weights, cable and cardio machines, kettlebells, and even a gear store — just the sort of variety that Rocks was looking for.

"I do two days of climbing per week and two days in the fitness area," he says. "Before this I was a member of a New York sports club, and I became just another jock, pumping weights, getting strong for no functionality. Here everyone wants to be a better athlete, fitter, healthier."

All around the country, tens of thousands of gym rats are giving up their Spinning classes and CrossFit WODs in favor of the climbing wall. According to the industry publication Climbing Business Journal, 29 climbing gyms — facilities with huge artificial walls studded with grips for indoor climbing — opened shop in 2014, and 40 more are expected this year. Walltopia, one of the leading manufacturers of climbing walls, has reported a 300 percent increase in business in the last four years alone.

"Climbing gyms used to be these dark, dirty places where only climbers would go," says professional climber Chris Sharma, a co-owner of Sender One, a glossy new gym in Santa Ana, California. "Nowadays pretty much everyone you talk to has tried climbing or knows someone who has."

A decade ago most climbing gyms attempted to re-create the look and feel of real rock. In the process, they constructed dusty, drab vaults that catered to hard-core rock hounds — and almost no one else. But that aesthetic has been turned on its head, and climbing gyms now attract urban professionals tired of the same old circuit-training class. Today's gyms are bigger, have more space and natural light, and are outfitted with multistory climbing walls painted with bright, eye-catching colors like orange, yellow, and green.

"It's never going to be real rock," says Sharma. "Accepting indoor climbing as something totally different has given gyms the freedom to finally create something that's inspiring."

Besides building airy spaces in neon colors, climbing gyms have added a full complement of yoga classes, functional-circuit boot camps, and kettlebell classes. In-house coffee bars and saunas are not uncommon, and some gyms also have creative work spaces and lounges. They host kids' birthday parties, after-school programs, and even late-night parties with DJs.

"It's the workout without the work," says Mike Wolfert, who opened the Cliffs in 2014. "You don't feel like you have to go to the gym. You actually want to go."

"Community is the secret to why climbing is so cool," says Lance Pinn, a co-owner of the Brooklyn Boulders (BKB) franchise, which added locations in Somerville, Massachusetts, and Chicago in the last two years. "Once people experience that, they get it. We're trying to become that third place people talk about — you know, other than work and home."

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All that new participation translates into an influx of money for the sport, and many mainstream brands have taken notice. Adidas Outdoor USA, a spin-off of the athletic-apparel brand, recently bought Five Ten, one of the most beloved brands in climbing, and now sponsors a team of 22 climbers that includes Kevin Jorgeson, who made worldwide headlines in January with his record-breaking ascent, with Tommy Caldwell, of Yosemite's Dawn Wall.

"There are about a thousand climbing gyms out there, and we think that will double in the next few years," says Greg Thomsen, managing director of Adidas Outdoor, which supplies a hundred of those gyms with employee clothing and artwork. "More important, they're arriving in areas where kids have only ever had access to sports like basketball."

How many of those new gymgoers will transition to climbing outdoors is hard to say. But plenty of elite climbers got their start in gyms, among them Jorgeson and Sharma. "Even though it's about being outside for me," says Sharma, "I feel connected to the climbing-gym world because that was my introduction to the sport. I'm the product of a climbing gym."

And when climbers do transition to the outdoors, it doesn't mean they stop coming to the gym. To train for the Dawn Wall, Jorgeson spent a huge amount of time indoors, and Caldwell even built a climbing wall in his backyard to problem-solve a particular move.

"Climbers in the past were more adventurers — they didn't work out, per se, other than climbing," says Thomsen. "But the new generation, they know they can't attain the level of performance they need without spending time in the gym."

"You're seeing more professional climbers doing focused indoor training to accomplish outdoor goals," says Jorgeson.

Thanks in part to the Dawn Wall ascent, climbing has suddenly hit the mainstream, in consciousness if not participation. And the boom, if it comes, will most certainly be felt in the gym first, not the outdoors.

"A lot of people were made aware of the sport because of that climb," Jorgeson explains. "But if you don't have a place to experience it, then it's just a headline and you're going to forget about it next week."

Of course, having an entire generation of climbers trained in the gym presents its own issues. "There are people starting in climbing gyms who don't know how to act when they go outdoors," says Sharma, who is opening a state-of-the-art gym in Barcelona, Spain, in July. But more climbers ultimately means a more sustainable future for the sport.

"I feel good about more people coming into climbing," says Sharma. "Our sport has been so small for so long, so it's great to see that people are psyched on it. I know what a positive thing that was in my life, and I would love for more people to have something similar."

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