Tsunami (津波 Japanese big wave in port), often incorrectly called a tidal wave, is a series of massive waves that occur after an earthquake, seaquake, volcanic activity, slumpss, or meteorite impacts in or near the sea. Since the constant energy of the tsunami is defined by height and speed, its height increases once its speed is reduced where the wave approaches land. The waves travel at high speed, more or less unnoticed where crossing deep water, but raising to a height of 30 metres and more. Tsunamis can cause severe destruction on coasts and islands.
Many cities around the Pacific, notably in Japan but also in Hawai'i, have warning systems and evacuation procedures in the event of a serious tsunami. A tsunami would be predicted by various seismologic institutes around the world and the progress monitored by satellites.
Evidence shows that megatsunamis, which are caused by significant chunks of an island collapsing into the ocean, are also possible.