On New Year's Day, 2005, David Shaw traveled halfway around the world on a journey that took him to a steep crater in the Kalahari Desert of South Africa, a site known as Bushman's Hole. His destination was nearly 900 feet below the surface. On January 8th he descended into the water. About fifteen feet below the surface was a fissure in the bottom of the basin, barely wide enough to admit him. He slipped through the opening and disappeared from sight,...