A tsunami, sometimes referred to as a tidal wave, is a series of ocean waves created when an earthquake or other major disturbance displaces a large volume of water. Powerful tsunamis such as the 2004 Indian Ocean event can devastate coastal regions as the waves sweep far inland.
Following a quake, tsunamis can travel for thousands of miles. For example, after the 2011 Sendai earthquake in Japan, a tsunami alert was issued for almost the entire Pacific region. Places as far away as Chile were affected by the alert.
A tsunami is often barely noticeable in deep ocean water, but as it approaches land and enters shallow water, the waves slow and increase in height.
Use the BBC News step-by-step tsunami animated guide to learn more.
Image: Thailand's coastline before (left) and after (right) the 26 December 2004 tsunami (credit: Geoeye/SPL)
Jem Stansfield uses a wave tank to explain tsunamis.
Jem uses a vice and a water tank to explain how the build up of stress between the Earth's tectonic plates is suddenly released, with sometimes devastating consequences.
Iain Stewart explains how an earthquake triggered the deadly 26 December 2004 tsunami.
Dr Iain Stewart explains how a megathrust earthquake at the bottom of the Indian Ocean triggered the deadly 26 December 2004 tsunami, which killed more than 225,000 people in 11 different countries.
What makes these massive waves so destructive?
Dr Iain Stewart explains why tsunami waves can be so much more destructive than ordinary storm waves.
Experts find shocking underwater evidence of the huge quake that caused the 2004 tsunami.
A team of experts searches for evidence of the huge underwater earthquake that generated destructive tsunami waves on Boxing Day 2004. They find a series of massive fractures at the bottom of the Indian Ocean that displaced large volumes of water and sent the tsunami waves on their way towards land.
Did a tsunami save Moses and his followers?
Dr Iain Stewart describes the theory that a volcano-triggered tsunami was the inspiration for the Bible story of God parting the Red Sea to save Moses and his Hebrew followers. There was a massive eruption at Santorini 3,500 years ago - around the same time that Moses and his followers fled slavery in Egypt, according to some scholars. If Moses really travelled through an area east of the Nile Delta once called the Reed Sea, as some experts believe, a volcano-triggered tsunami in the Mediterranean may have first drained away the water and then flooded the coast, drowning the pursuing pharaoh's army. This clip contains footage provided by ITN.
A tsunami (plural: tsunamis or tsunami; from Japanese: 津波, lit. "harbour wave"; English pronunciation: // or //) , also known as a seismic sea wave or as a tidal wave, is a series of waves in a body of water caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami. In being generated by the displacement of water, a tsunami contrasts both with a normal ocean wave generated by wind and with tides, which are generated by the gravitational pull of the moon and the sun on bodies of water.
Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason they are often referred to as tidal waves. Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called "wave train". Wave heights of tens of metres can be generated by large events. Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous and they can affect entire ocean basins; the 2004 Indian Ocean tsunami was among the deadliest natural disasters in human history with at least 290,000 people killed or missing in 14 countries bordering the Indian Ocean.
The Greek historian Thucydides suggested in his late-5th century BC History of the Peloponnesian War, that tsunamis were related to submarine earthquakes, but the understanding of a tsunami's nature remained slim until the 20th century and much remains unknown. Major areas of current research include trying to determine why some large earthquakes do not generate tsunamis while other smaller ones do; trying to accurately forecast the passage of tsunamis across the oceans; and also to forecast how tsunami waves would interact with specific shorelines.