Dec. 28, 2004  -- The recent natural disaster in Southeast Asia underscores the power of tsunamis. The following are some facts about tsunamis, their history and the warning systems that can protect coastal residents.

A tsunami is an ocean wave, or a series of waves, caused by a sudden physical disturbance in the ocean such as an earthquake.

Landslides, volcanic eruptions, even meteorites can cause tsunamis.

The word "tsunami" comes from the Japanese words meaning "harbor wave."

Tsunamis differ from tidal waves, which are caused by the action of planetary bodies like the moon.

Tsunamis can travel thousands of miles across the open ocean, where they are difficult to see, rarely reaching more than 3 feet in height.

Tsunamis often exceed 100 miles in length in the deep ocean, where they can travel as fast as 500 miles per hour, crossing the entire Pacific Ocean in less than 24 hours.

As a tsunami reaches the shallow water near a coastline, its speed decreases, but as the wave compresses it may grow to a height of 20 feet to 30 feet or more.

The height of a tsunami as it approaches a shoreline is referred to as its "run-up."

The run-up of the 1964 tsunami that struck Alaska was as high as 90 feet (27.4 meters) in some places.

Many tsunamis do not result in dramatic, giant waves -- they come in like very strong, fast high tides that cause extensive flooding.

Tsunamis are often preceded by a rapid drop in local sea level, similar to an unusually low tide.

Tsunamis, and the geologic events that cause them, are some of the most severe natural disasters known.

About 36,000 people died in the 1883 eruption of Krakatoa and the subsequent tsunami. In 1896, a tsunami with a run-up of about 100 feet swept the east coast of Japan; more than 27,000 people were killed. A 1908 earthquake and tsunami in Sicily caused an estimated 58,000 deaths.

A number of national and international organizations cooperate to provide tsunami information and early warnings of tsunamis, including the United Nations and the National Oceanic Atmospheric Administration.

Many communities on the Pacific Coast of the United States participate in early warning systems like the West Coast/Alaska Tsunami Warning Center and the Pacific Tsunami Warning Center in Hawaii.

Sources: University of Washington Department of Earth and Space Sciences, United States Geological Survey, National Oceanic and Atmospheric Administration