A SMALL, nondescript door on a dark, industrial block in Brooklyn opens onto an enchanted world.

Brightly speckled boulders, soaring arches, music, clapping, peace, harmony and free bike parking abound. Adults and children hang from the ceilings like bats and stretch like spiders across the walls.

Brooklyn Boulders, as the cavernous, 22,000-square-foot playground is known, is the city’s largest and possibly most popular rock-climbing gym. But it is more than just a destination for adventure athletes.

Only a little more than two years after it opened, in a former Daily News truck garage off Third Avenue in Gowanus, the gym has become a cozy sanctuary, embracing all ages and skills.