A tsunami is a chain of fast moving waves in the ocean caused by powerful earthquakes or volcanic eruptions. Tsunami has a very long wave. It can be hundreds of kilometers long. Usually, tsunami starts suddenly. The waves travel at a great speed across an ocean with little energy loss. They can remove sand from beaches, destroy trees, toss and drag vehicles, damage houses and even destroy whole towns.
The water will draw back from the coast half of the wave period prior to the wave getting to the coast. If the slope of the coast is not deep, the water may pull back for hundreds of metres. People who do not know of the danger will often remain at the shore.
Tsunamis can not be prevented. However, there are ways to help stop people from dying from a tsunami. Some regions with lots of tsunamis may use warning systems which may warn the population before the big waves reach the land. Because an earthquake that caused the tsunami can be felt before the wave gets to the shore, people can be warned to go somewhere safe.