The effects of the Indian Ocean tsunami of December 2004 are only too well known: It knocked the hell out of Aceh Province on the Indonesian island of Sumatra, leveling buildings, scattering palm trees, and wiping out entire villages. It killed more than 160,000 people in Aceh alone and displaced millions more. Similar scenes of destruction were repeated along the coasts of Southeast Asia, India, and as far west as Africa. The magnitude of the disaster shocked the world. What the world did not know was that the 2004 tsunami—seemingly so unprecedented in scale—would yield specific clues to one of...