The worst tsunami disaster in history occurred in December 2004 when a magnitude 9.0 undersea earthquake, centered in the Indian Ocean off the northwestern coast of the Indonesian island of Sumatra, generated a tsunami that struck the coasts of 14 countries from Southeast Asia to northeastern Africa.

The International Committee of the Red Cross reported a death toll of more than 250,000 people as a result of the tsunami and the earthquake, with nearly two-thirds of the deaths occurring in Indonesia. High death tolls were also reported in India, Sri Lanka, and Thailand. Geologists calculated that the ocean floor at the epicenter was thrust upward 9 m (30 ft) as a result of the quake. Previously the highest death toll from a tsunami was an estimated 60,000 people killed in 1755 when an earthquake generated a tsunami that struck coastal Portugal, Spain, and Morocco and destroyed the city of Lisbon, Portugal.

The last major tsunami to occur in the Indian Ocean happened in 1883 with the eruption of Krakatau (also spelled Krakatoa). The resulting tsunami reached an estimated height of 30 m (100 ft), traveled 13,000 km (8,000 mi), and drowned about 34,000 people along the coasts of Java and Sumatra. Another 2,000 people were fatally burned by hot volcanic ash.

In North America the worst known tsunami occurred in 1964 when an earthquake off the coast of Anchorage, Alaska, created a tsunami that killed 115 people in Alaska, Oregon, and California. Scientists also believe that a magnitude 9.0 quake occurred along the Cascadia fault off the coast of Washington and Oregon in 1700 and generated a massive tsunami.

Scientists believe the quake and tsunami converted vast spruce tree forests into saltwater tidal flats.