In 1989, Edward Bryant climbed a point on the southeast coast of his native Australia with a colleague and found an odd jumble of boulders well above the surf. A big wave, he thought, maybe a tsunami from an earthquake, must have tossed them up there. Over the next few years, however, the University of Wollongong geologist explored hundreds of miles of coast and found more signs of wave action, hundreds of feet above the water--too high for any quake-spawned surge. An astonishing hypothesis of devastation from outer space formed in his mind. It gathered some praise, along with many...