A series of very large ocean waves which are caused by large disturbances such as an underwater earthquake or volcanic eruption, or meteorite impacts from above. Tsunami waves are of extremely long length and period and they move outwards, away from their source. This allows time for the tsunami to grow in wave height. As a tsunami crosses a deep ocean, the length from crest to crest may be as much as 150 km but the height may be less than a metre. Tsunami waves may therefore be unnoticed by ships or from the air, reaching speeds up to 1000 km per hour.