"No boys allowed," proclaims The Underground's website. If that makes it sound more like a playhouse than a gym, that's not an accident. Inside, you won't find any rows of treadmills or the latest elliptical machines. Instead, lights fit for a dance club shine as catchy music plays through an advanced sound system. In this festive space, women of all fitness levels support each other as they throw themselves into a wide range of classes, from the Latin dance–inspired cardio workouts of Zumba to the intense cross-training regimen of P90X. In other parts of the studio, they sculpt muscles via Booty Barre classes, learn to swing from aerial equipment, and work their way up dance poles in hopes of finding the hunky fire fighters at the top.