Tsunamis are caused by earthquakes, landslides or volcanic eruptions that disturb the water and send a series of waves that can travel for thousands of kilometres and gather in size.
The effect of these geological disturbances on the ocean is similar to the ripples caused by a stone being dropped in a pond.
When a tsunami hits a coastline they grow in height as they are pushed up by the shallower ocean floor and can reach 30m in size.
Walls of water
A tsunami wave can be more than 100km in length and can continue for more than an hour, as opposed to ordinary wind-generated swells that on average last 10 seconds and have a wave length of 150m. Some can travel at speeds of 600 miles an hour - as fast as a passenger jet.
In some cases, the sea can recede before a tsunami, leaving empty harbours and fish flopping on the mud. This can draw curious people to the shoreline, worsening the death count when the wave hits.
Survivors describe tsunamis as dark walls of water which can cause devastation as they sweep over coasts, destroying houses, picking up cars and snapping trees.
The swell can destroy buildings,
pick up cars and snap trees
Some recent disasters caused by tsunamis:
1946: An earthquake in the Aleutian islands sent a tsunami to Hawaii, killing 159 people.
1964: An Alaskan earthquake triggered a tsunami up to six metres tall that killed 11 people as far away as California, and caused more than 120 deaths in all.
1983: 104 people in western Japan were killed by a tsunami caused by a nearby earthquake.
On 17 July 1998: A Papua New Guinea tsunami killed approximately 3000 people. A 7.1 magnitude earthquake 15 miles offshore was followed within 10 minutes by a wave about 12m tall.