A tsunami is a set of waves triggered when a body of water is quickly displaced by an occurrence such as an earthquake, volcano, underwater explosions or landslide.

1. From the Japanese: tsu (harbor) nami (wave)
2. Warning Sign: rapidly receding shoreline
3. Fast and low across open water
4. Slower and high across shallow water
5. Is more than one wave
6. Not a tidal wave
7. Duration: several hours

A tsunami is typically the result of an undersea earthquake. In the 2004 Indian Ocean tsunami, a megathrust earthquake caused the ocean floor to rise by several meters. This rise in the ocean floor displaced the water above it, causing a higher sea level in the immediate earthquake zone than in the area further away. With no place to go, the water began to flow to the area of least resistance, causing a massive tsunami. Over open water, a tsunami typically moves very fast, and can often not be seen by the naked eye because most of the force is occurring beneath the surface of the ocean. However, once a tsunami nears shore, where the ocean floor becomes shallower, the force of the wave will cause water near shore to rush temporarily out to sea. However, this is only temporary. The approaching tsunami then begins to rise, sometimes to heights of dozens of meters. The initial crash of the wave, however, is not what causes the most destruction. Instead, it is the enormous amount of water behind the initial wave which causes the most deaths, because the wave, unlike normal waves, does not recede quickly back to sea level, but often can travel miles inland, sweeping people, homes, and debris in its wake.