These huge waves, labelled megatsunamis, happen when a landslide caused by either an earthquake or loose rock hit water below. They are usually localised and dissipate quickly.
The other type of tsunamis happen in the open ocean and are directly triggered by earthquakes, or occasionally other powerful events such as underwater volcanoes.
They, like the 2011 Japan tsunami, can travel for thousands of miles, hitting populated areas and causing widespread devastation and loss of life.
The biggest megatsunami was in the 1950s and was over 500 metres. This latest megatsunami was the second largest.
