climbing the walls - View this location on map bouldering, the sport of performing balletic rock-climbing moves to ascend boulders without a rope, is exploding in popularity, and perhaps the best place to learn it is the spot bouldering gym (303-379-8806), one of the largest such gyms in the world. one free-standing artificial boulder mimics the contours and overhangs of the sandstone at hueco tanks in texas; another resembles that at fontainebleau in france. in all, the gym has over 100 climbing ''problems'' of varying difficulty. when you fall -- and you will -- the gym's floor has the springy air cushion of a running shoe to soften the blow to bones, if not egos ($10 for a first-day pass, $14 thereafter). New York Times 3240 prairie avenue, Boulder, CO Active, Kids

---