Movements of plates in the top layers of the Earth’s crust create enormous tension, which sometimes makes long cracks, or faults, in rocks. Continuing tugging movements make blocks of land sink down between roughly parallel sets of faults, forming troughs called rift valleys.
The world’s biggest rift valley runs from south-eastern Africa, through East Africa and the Red Sea to Syria. This long gash in the Earth contains the Dead Sea, a lake whose shoreline is 400m (1,312ft) below sea-level. This is the world’s deepest depression.