The first twelve hours of the 2004 Indian Ocean tsunami caused by a 900-mile long submarine earthquake (indicated by the stars). But it was the first two hours that were most devastating.Only 17 minutes after the earthquake, a 100-foot tsunami bulldozed towns out of existence along the Aceh coast of northern Sumatra, Indonesia. In less than an hour 240,000 were dead. In less than two hours smaller but still powerful tsunami waves had killed 7,500 in Thailand, 31,000 in Sir Lanka, and 16,000 in India. Submarine ridges focused the tsunami wave energy like a lens focuses light, eventually guiding them out of the Indian Ocean and up the Atlantic, though now greatly reduced in size. It was that same bathymetric effect that sometimes determined who would live and who would die. More than 8,500 Sri Lankans died in Kalmunai at the shoreward end of a submarine ridge, while eight miles south only 2 died in Oluvil at the shoreward end of a submarine canyon. This is a modified version of a model-produced map from NOAA’s National Geophysical Data Center.