The Strait of Hormuz. To the right of the image is the Gulf of Oman; to the left is the Persian Gulf.

The Straits of Gibraltar or which separate Africa and Europe have been used by shipping for several thousand years and in classical times were regarded as marking the western extreme of the world: they were called by the latin name Fretum Herculeum which means Pillars of Hercules. The strait is 58 km (36km) long and narrows to 13 km (8 km) between Point Marroquí in Spain and Point Cires in Morocco.

The Strait of Gibraltar photographed from the Space Shuttle.

Straits can be of great economic and strategic importance, since they are often important shipping routes. The Dover and Gibraltar Straits connect Europe are among the busiest waterways in the world, while the Hormuz Strait provides access to the Persian Gulf for supertankers collecting oil from ports of the Gulf states.

Straits are often characterised by very strong currents because of the large volumes of water which can be pushed rapidly through a relatively narrow neck by tides. Sometimes the flow is due to an ongoing exchange between two bodies of water. Through the Strait of Gibraltar there is a two-knot surface current which flows eastwards. At a depth of about 120 metres (320 feet) a cold saline stream of water flows westward out of the Mediterranean. Studies have found that more water enters than leaves. The disparity shows that if it were not for the strait, the Mediterranean Sea would become an inhospitable salt lake, eventually drying up completely.