Tucked behind Union Station, Upper Limits has approximately 10,000 square feet of climbing, and although the wall isn't the highest -- about 35 feet -- set routes run from near-vertical to seriously overhanging, and the textured concrete walls could pass for real rock, were it not for the multicolored climbing holds that rise to the ceiling. If you don't have a climbing partner and aren't interested in bouldering (climbing without a rope, usually no more than ten to twelve feet off the ground), use one of Upper Limits' three auto-belay devices to climb safely by yourself. You have to get certified to climb, so if you're new to the sport, make sure to call ahead and sign up for one of the three Rock Gym 101 courses offered every week ($30). It's cheaper to climb during the week than during the weekend, but either way, it's a pretty good deal. For those who don't have their own gear, Upper Limits rents shoes, harnesses and belay devices. For a special treat, stay late and listen to music thumping through the walls from the nightclub next door.