1Most people assume that powerful earthquakes only ever happen somewhere on dry land.
2In fact, many of the strongest earthquakes ever recorded actually begin deep under the ocean.
3The floor of the sea is made of enormous rocky plates that never stop moving.
4These giant plates push against each other extremely slowly, moving only a few centimetres every year.
5Sometimes the pressure between two plates becomes too great, and one of them suddenly slips past the other.
6This sudden movement releases a shock that shakes the entire column of water above the sea floor.
7The released energy then travels across the open ocean in the form of long and powerful waves.
8In deep water, these waves are surprisingly low and almost impossible for ships to notice.
9However, as the waves reach the shallow water near a coast, they slow down and rise up.
10They can grow into terrifying walls of water that stand many metres above the normal sea level.
11To reduce the danger, scientists use special sensors resting on the ocean floor to detect these movements.
12Their early warnings can give people in coastal towns precious time to escape to higher ground.