Tsunamis: Facts About Killer Waves

Within hours killer waves radiating from the epicenter slammed into the coastline of 11 Indian Ocean countries, snatching people out to sea, drowning others in their homes or on beaches, and demolishing property from Africa to Thailand.

Tsunamis have been relatively rare in the Indian Ocean. They are most prevalent in the Pacific. But every ocean has generated the scourges. Many countries are at risk.

In the wake of the Christmas weekend tsunami in the Indian Ocean, one of the worst disasters in history, National Geographic News examines the killer waves' causes and warning signs—information that can be a lifesaver in a tsunami zone.

• A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. More rarely, a tsunami can be generated by a giant meteor impact with the ocean.

Scientists have found traces of an asteroid-collision event that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the mountains 3.5 billion years ago. The coastline of the continents was changed drastically and almost all life on land was exterminated.