WHEN tsunamis hit a coastal area, they often cause substantial damage and loss of life at locations otherwise well protected from normal (even severe) wind-generated waves. An example of such a catastrophe took place on 12 December 1992, when an earthquake of magnitude Ms = 7.5 occurred in the eastern region of Flores Island, Indonesia1,2. One of the areas hardest hit by the resulting tsunamis was Babi, a small coastal island; two villages on the island were completely destroyed, even though both were located in sheltered areas where wave conditions are normally calm. Here we describe the results of numerical and laboratory experiments that show how, unlike wind-generated waves, tsunamis are capable of penetrating into sheltered coastal areas without significantly losing their energy. For the case of Babi Island, the original tsunami wave was split into two, with one wave propagating around each side of the island. The two waves met in the sheltered region, and the subsequent amplification of wave amplitude resulted in the destructive flow onto the beach.