Tsunami

A tsunami is a series of long-period gravity waves set off by a sudden vertical displacement of the seafloor, usually a subduction-zone earthquake but also a submarine landslide, volcanic collapse, or meteorite. In the deep ocean the wave is under a meter high but hundreds of kilometers long, traveling at jet-airliner speed (about 700 km/h in 4,000 m of water); shoaling at the coast piles it into a destructive surge. Warning relies on seismic and DART pressure-sensor networks.