Underwater explosions create an enormous expanding bubble filled with scorching gases. The growth eventually loses momentum, and the water pressure squeezes the gassy globule back down. The internal pressure rebuilds and the gas and water pressure continue this tango, cyclically expanding and contracting the walls of the gas pocket as it rises Hydroacoustic Station to the surface. According to Lay, a few microphones stationed in the major bodies of water are all that are needed to listen for this "bubble pulse." Hydrophones, as they are called, consist of a pressure sensitive floating two-foot long cylindrical bump of on a hose that can feel sound waves. "Sound travels extremely well in the ocean," says Lay. "Even small chemical explosions are extremely loud. An underwater nuclear explosion is likely to bust everyone's eardrums." But that does not mean that there won't be any false alarms.