The largest tsunami of recent times was on December 2004, just off the coast of Indonesia. The deaths of more than 250,000 people were caused by nature. This was nothing unusual in the four billion years of Earth’s history. During this time the Earth has developed a thin, solid, brittle crust. It is the rock we live on. But below it lies material that is hot and continually on the move. This is the source of earthquakes and volcanoes which in turn can cause tsunamis.

The layer below the crust is called the mantle. It is like a sticky, hot plastic. It moves at just a few centimetres a year. But it is so powerful that it simply drags the crust with it. The crust is brittle and cannot move easily, so stresses build up as the mantle tugs at the crust. Eventually the crust gives way and snaps.

The snap sends out shock waves and the shock waves produce an earthquake. At the same time, the snapping rock moves up or down. Huge slabs of crust can move within seconds. If this happens under the ocean, it lifts the sea upwards into a dome of water. The sea water then begins to flow away from the centre of this dome, creating a series of waves.

The waves contain billions of tonnes of water. Yet out at sea they are not noticeable. A typical tsunami is half a metre high and 100 kilometres wide, but it moves at over 600 kilometres an hour. The energy in this water is beyond imagination. Throw a bottle of water (a litre weighs a kilo) at a chair and you will knock it over. Then try to picture the result of billions of tonnes of water moving 100 times faster. In the ocean the tsunami is hardly noticed. But as soon as the waves reach the coast all of the energy stored in the water is used in smashing up the land, plants, buildings and people who might be close to the sea.

Tsunamis cause more deaths in modern times because so many people now live or go on holiday by the coast.