A series of waves in a body of water (ocean, sea, lake, etc) that wreaks havoc on the
coastlines of said body of water. Caused by displacement of the water, like throwing a rock into a pond; mostly caused by earthquakes, landslides, volcanic eruptions, the occasional asteroid/comet, anything that disrupts the water.
Unlike ordinary waves, tsunami waves act more like shock waves from an explosion; they affect the whole water column from the surface to the sea floor and in the open sea travel as fast as an airliner.
In deep water, they hardly look different from normal waves, but as they approach the shore they slow down, their energy is compressed, and they turn into monsters: the water often drains from the beach, exposing the sea floor and
stranding boats and
sea animals (people should know that this is a warning to run like hell for
high ground); then the first of a series of increasingly huge waves batter the coastline and sweep people inland, one wave coming in just as the previous one is draining away; finally, the waves drain back to sea, dragging debris and people with them and leaving the coast a mass of wreckage.
The Japanese are regularly visited by tsunami. The evidence for this is the word itself, which means "harbor wave" in Japanese, referring to the great damage that the waves do to ports (among other things). But even the Japanese are taken by surprise; the 2011 Tokohu earthquake and tsunami proved tsunami barriers useless (many of the 15,000 victims died because of a false sense of security) and nearly created the next Chernobyl.
The
Minoan civilization on
Crete was severely crippled by many tsunami around 1600 BC when
Thera (
Santorini) blasted itself 20 miles into the sky.
