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Last Updated: Tuesday March 15 2011 10:46 GMT

WATCH: How tsunamis happen

The earthquake that struck Japan triggered a tsunami which swept away whole towns.

Earth's surface is made up of plates and huge amounts of energy can build up when they rub together.

An earthquake happens when the energy is suddenly released, and if that's under the sea it can cause massive waves.

Jem from Bang Goes The Theory explains how...

Bang Goes The Theory is back on BBC One on Mondays at 7.30pm.