Tsunamis are caused by earthquakes. When an earthquake occurs under the sea, it sometimes produces a circular motion of water, perhaps 1 km in diameter. This impulse then spreads out from this epicentre, can travel up to 800 km per hour, and speeds through the open water virtually unnoticed. Under the right conditions, this disturbance in the water forms a very definite waveform with incredible power behind it. As it enters shallow water, it forms a large destructive wave called a "tsunami".