By day, Gillham Park Pool is an urban oasis, a free swimming hole for midtown residents. Open sporadically, with unposted hours that apparently depend on the availability of the teenage lifeguards, the pool gets so packed that only slim streams of water separate the shoulder-to-shoulder bodies. Each afternoon, though, the lifeguards pull the plug, foiling late-night skinny dippers and protecting unwatched children who might wander there after hours. When the pool runs dry, skateboarders take flight, and the aerial antics continue late into the summer nights. A replica of this multilayered concrete structure could be a model centerpiece for Kansas City's long-discussed public skatepark.