Among the dangers of free diving, the most disconcerting is shallow-water blackout--the brain's frightening tendency to shut down within 15 feet of the surface during the ascent. As you descend, water pressure squeezes your lungs, condensing the oxygen and giving you what feels like a second breath. During the return trip, however, your lungs re-expand, dissipating what's left of your oxygen. If levels drop too low, not enough will move into the bloodstream, and the lights go out. Fortunately, the body's laryngospasm reflex kicks in to tighten the throat and keep water out for up to a minute--just enough time for your dive buddy to drag you to the surface, tilt your head back, and beg, "Breathe, baby."