A tsunami (say it like this: soo-nah-mee) is a series of large waves that are different from normal waves. A tsunami is not caused by wind.

Most tsunamis are caused by an earthquake at the bottom of an ocean. Some tsunamis happen when an underwater volcano erupts. Both of these events can make large amounts of water start moving, and a tsunami is formed.

A tsunami wave is not very tall when it first forms in the deep water. As the wave moves closer to the shore, it gets taller and taller. The tallest tsunami we know about grew to be about 530 metres tall. That tsunami happened in Alaska in 1958.

Tsunamis travel very fast when they are in deep water. They slow down when they reach shallow water. When a tsunami is moving fast, it can travel at the same speed as a jet plane!

In 2011, there was a very powerful earthquake under the ocean near Japan. The earthquake started a tsunami. Some of the waves grew to over 40 m tall when they reached Japan. After waves reached the shore, they travelled as far as 10 km across land.

Right after the earthquake, people in Japan were warned that a tsunami was coming and they should get to high ground. Most people who paid attention to the warning were safe. Many people who did not pay attention were killed by the waves.