Bouldering – unroped rock climbing on walls no higher than 15 feet – is exploding right now. Hold-studded walls are drawing crowds into more than 600 climbing-specific gyms in the United States, and competitions are attracting a higher caliber of athlete. Still, even though first timers need only a pair of shoes, bouldering's barrier to entry remains surprisingly high. This is the most challenging climbing discipline, and it requires a potent combination of technique and strength. "A lot of people walk into a climbing gym for the first time and think the bouldering area is the kiddie wall," says Luke Livesey, director of curriculum at Brooklyn Boulders climbing gym in New York City. "That couldn't be further from the truth."