A tsunami is a wave caused by earthquakes or undersea volcanic eruptions

This series of three images shows the progression of the Sumatra Tsunami on December 26, 2004. This tsunami was the result of a massive earthquake that occurred when the India tectonic plate subducted beneath the Burma plate, causing an earthquake with a magnitude of at least 9.0 on the Richter scale. The earthquake displaced a huge amount of water, creating waves with heights of 10.8 meters (35.5 feet). The Sumatra Tsunami travelled around the world in just one day, devastating coastal communities and killing an estimated 230,000 people.

Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these giant waves.

The largest tsunami on record rushed past Ishigaki Island, Japan, in 1971. It was an incredible 84.7 meters (278 feet) high. While it caused little damage, the giant wall of water relocated a 750-ton block of coral 2.4 kilometers (1.5 miles) inland.

For more information:

NOAA Deep Water Assesment and Reporting of Tsunamis (DART)
Map and Real-time DART Data
NOAA Tsunami Program
National Geophysical Data Center
NOAA Enhances Its Ability to Provide Tsunami Warnings (NOAA 200th Anniversary Web site)