Mediterranean Sea, inland sea of Europe, Asia, and Africa, linked to the Atlantic Ocean at its western end by the Strait of Gibraltar. Known to the Romans as Mare Nostrum (our sea), the Mediterranean is almost landlocked. It is of great political importance as a maritime outlet for the countries of the former Union of Soviet Socialist Republics, via the Bosporus, Sea of Marmara, Dardanelles, and Black Sea, and for European and American access to the petroleum of Libya and Algeria and the Persian Gulf region, via the Suez Canal and overland pipelines.

The Mediterranean Sea covers an area of 2,509,000 sq km (969,000 sq mi). It has an eastern to western extent of 3,900 km (2,400 mi) and a maximum width of 1,600 km (990 mi). Generally shallow, with an average depth of 1,500 m (4,900 ft), it reaches a maximum depth of 5,150 m (16,896 ft) off the southern coast of Greece.

The Mediterranean is a remnant of the vast ancient sea called Tethys, which was squeezed almost shut in the Oligocene Epoch, 30 million years ago, when the crustal plates carrying Africa and Eurasia collided (see Plate Tectonics). The plates are still grinding together, causing the eruption of volcanoes such as Mount Etna, Mount Vesuvius, and Stromboli, all in Italy, and triggering frequent earthquakes, which have devastated parts of Italy, Greece, and Turkey.

An undersea sill from Tunisia to Sicily divides the Mediterranean into eastern and western basins. Another seafloor sill, from Spain to Morocco, lies at the outlet of the Mediterranean. Only 300 m (1,000 ft) deep, it restricts circulation through the narrow Strait of Gibraltar, thereby greatly reducing the tidal range of the sea and, coupled with high rates of evaporation, making the Mediterranean much saltier than the Atlantic Ocean.

Malta and Sicily have commanded shipping through the strategically located Straits of Sicily and Messina. Other important islands include the Balearic Islands (Spain); Corsica (France); Sardinia (Italy); Cyprus; and the Ionian, Cyclades (Kikládhes), Dodecanese, and Aegean islands (Greece). Arms of the Mediterranean include the Tyrrhenian Sea (located off western Italy), the Adriatic Sea (between Italy and the Balkan Peninsula), and the Aegean and Ionian seas (off peninsular Greece). Barcelona, Marseille, Genoa, Trieste, and Haifa are important seaports in the region. Major rivers entering the Mediterranean are the Ebro, Rhône, Po, and Nile.