Illustration showing the formation of a tsunami following an underwater earthquake (top). The seabed has moved upwards and the displacement of the water above the seabed has produced a series of powerful waves (concentric circles). These waves move rapidly (hundreds of kilometres an hour) in the open sea. As they approach a shoreline, they slow down and bunch up, sometimes towering tens of metres above the original sea level. A large mass of water crashes ashore as a tsunami, causing immense damage and destruction, and then dragging debris and bodies out to sea.