How do tsunamis form?

Tsunamis are giant sea waves usually triggered when an undersea earthquake abruptly shifts the ocean floor, displacing an enormous volume of water. Underwater landslides, volcanic eruptions and even asteroid impacts can also cause them. In the deep ocean a tsunami may be barely noticeable, only a metre or so high but travelling as fast as a jet aircraft, around 800 kilometres per hour. As it approaches shallow coastal water it slows down and its energy piles up, so the wave grows dramatically in height. A tell-tale warning sign is the sea suddenly retreating far from shore just before the wave strikes. The 2004 Indian Ocean tsunami killed around 230,000 people.