A tsunami is a series of waves caused by the displacement of water, usually in an ocean, which is often triggered by earthquakes, volcanic eruptions, and other underwater explosions. Tsunamis have small wave heights offshore but grow taller as they reach shallow coastal waters, posing a threat to areas near the shore. Notable deadly tsunamis include the 2004 Indian Ocean tsunami that killed over 130,000 people and the 2011 Japan tsunami that caused over 9,500 deaths.