"Tsunamis are a series of very long waves generated by any rapid, large-scale disturbance of the sea. Most are generated by sea floor displacements from large undersea earthquakes. Tsunamis can cause great destruction and loss of life within minutes on shores near their sources, and some tsunamis can cause destruction within hours across an entire ocean basin." from NOAA Tsunamis
On December 26th, 2004 a powerful earthquake registering 9.0 on the Richter scale occurred deep in the ocean off the northwest coast of the island of Sumatra. The earthquake triggered massive tsunamis that severely affected more than 12 countries including Indonesia, Malaysia, Thailand, India, Sri Lanka, the Maldives, Kenya, Myanmar, Somalia, Kenya, Bangladesh and the Seychelles.