A tsunami in common usage is an ocean wave generated by earthquakes or other geological events, including asteroid impacts, under the sea or related collapses of continental shelves. A tsunami is preceded by a stage known as 'drawback' during which the coastal waters may recede for hundreds of metres before the tsunami arrivesTechnically they are in fact a series of waves. Tsunamis have great destructive power, and may travel across oceans before making landfall. Some of the most destructive tsunamis occur in the areas adjacent to major earthquakes.