Mediterranean Sea, so named from lying in the midst between the continents of Europe, Asia, and Africa. It is the largest enclosed sea in the world, and is connected with the open ocean only by the narrow Strait of Gibraltar, 9 miles in width at the Pillars of Hercules. Since 1869, however, it has been artificially connected with the Red Sea and Indian Ocean by means of the Suez Canal (q.v.). From its great size the Mediterranean might be ranked with the oceans, but from being so completely cut off it presents distinctly local characters when compared with the great ocean-basins, and is consequently of special interest to the student of physical science. The Mediterranean, in a nearly east and west direction, is about 2200 miles in length from the Strait of Gibraltar to the Syrian coast; its width varies from 500 or 600 miles in some places to less than 100 miles between Sicily and Cape Bon, where it is divided by relatively shallow banks into two distinct hydrographic basins, the eastern one being the larger. It is connected with the Black Sea through the Dardanelles, the Sea of Marmora, and the Bosphorus. The African and Syrian coasts are comparatively even and unindented, the wide gulfs of Gabes and Sidra scarcely presenting an exception; on the other hand, the shores of Europe and Asia Minor are cut up into numerous gulfs and bays, the largest of which is the Adriatic Sea. Various parts of the Mediterranean have been known by special names, such as the Tyrrhenian and Iberian Seas in the western, and the Levant, Ægean, and Ionian Seas in the eastern basin. The principal islands in the western part are Sardinia and Corsica, the Balearic and Lipari Islands, the two latter groups being of volcanic origin. The continental islands of Sicily and Malta are situated on the banks dividing the two basins; Pantellaria, Limosa, and Graham Island (now reduced to a shoal) are, however, volcanic though situated on the same banks. In the eastern regions there are the large islands of Cyprus and Crete, with the Ionian Islands and the islands of the Archipelago. The Mediterranean is frequently subject to earthquakes, and Vesuvius, Stromboli, and Etna are among the most famous of its active volcanoes. The scenery of the shores of this great inland sea is varied, mountain-ranges and high tablelands predominating. Although there is considerable diversity between the climates of Northern Italy and the desert shores of North Africa, still the terrestrial fauna and flora are not markedly distinct in the different regions of the Mediterranean basin, many of the plants and animals being identical on the northern and southern shores, and there is abundant evidence that this similarity was much more pronounced in recent geological times. The countries bordering the Mediterranean have been the cradle of civilisation, Phœnicia, Greece, and Italy having been successively the homes of knowledge and progress, and at the present time this inland sea is commercially the most important waterway of the world.
The area of the Mediterranean is estimated at about 900,000 sq. m., or, including the Black Sea and Sea of Azov, at 1,053,000 sq. m. The area of land draining into the Mediterranean is estimated at 2,969,350 sq. m., or nearly 3,000,000 sq. m. of the richest country on the earth's surface. The annual amount of rain that falls on this land is estimated by the writer at 1598 cubic miles, and of this amount about 226 cubic miles reach the Mediterranean through the annual discharge of rivers, the principal of which are the Rhone, Po, Danube, Dnieper, Don, and the Nile.
The basin of the Mediterranean commences about 50 miles to the west of Gibraltar, where there is a ridge with a maximum depth of about 200 fathoms. There is a similar depth on the ridge between Sicily and Africa which separates the Mediterranean into two basins: 2040 fathoms is the greatest depth recorded in the western, and 2187 fathoms the greatest in the eastern basin; the mean depth of the whole sea is 768 fathoms. The area of the sea-bottom with a less depth than 100 fathoms is estimated at about one-fourth of the whole area; the area with a depth of from 100 to 1000 fathoms is estimated at 300,000 sq. m., and with a depth of from 1000 to 2000 fathoms at 15,000 sq. m. The bulk of water is estimated at 709,800 cubic miles. The greatest depth in the Black Sea is 1070 fathoms, the average depth being 412 fathoms.
On the whole northerly winds prevail over the Mediterranean, due chiefly to the influence of the anticyclonic region of the North Atlantic, although in the eastern portions the alternate cyclonic and anticyclonic area of northern Asia has a distinct influence on the direction of the winds. The Mediterranean lies wholly between the annual isotherms of 60° F. in the north and 70° in the south. The temperature of the surface waters may occasionally reach 90°, but is usually much less, the mean of the winter months being between 53° and 57°. Generally the temperature of the sea is higher than that of the air, especially in winter, but in some of the summer months the reverse is the case. Whatever the temperature of the surface water may be, at a depth between 100 and 200 fathoms a temperature of 54° to 56° is met with, and this persists without sensible variation to the greatest depth. The temperature of the bottom water in the western basin is about 54°·5, and in the eastern basin a little warmer, 56°·0, these temperatures being fully 20° higher than the temperature of the bottom water of the Atlantic at corresponding depths. From recent observations it would appear that the deep water of the Mediterranean is subject to slight annual variations, dependent on the temperature of the previous winter. The evaporation from the surface of the Mediterranean exceeds both the precipitation and the annual discharge of the rivers flowing into it from the surrounding catchment basin, for we find the specific gravity of its waters (1.02800 to 1.0300) to be greater than that of the Atlantic on the west (1.026 to 1.027), or that of the Black Sea on the east (1.012 to 1.014). There is even an outflow of warm dense Mediterranean water into the Atlantic beneath the lighter Atlantic water which flows in at the surface through the Strait of Gibraltar. There is a similar state of things at the entrance of the Black Sea, where there is an inflow of fresh water from the Black Sea at the surface, and an outflow in the opposite direction of salter Mediterranean water by an undercurrent. Were it not for the inflow of Atlantic water the Mediterranean would slowly become salter, and shrink till reduced to two salt lakes like the Dead Sea. The Mediterranean is usually called a tideless sea. At Algiers there is a rise of 3½ inches at springs and half that amount at neaps; at other places the rise and fall is about 18 inches, and in the Gulf of Gabes the range reaches 5 feet, but the solitunar tides are as a rule completely masked by the rise of level and the surface currents produced by the action of the winds. The deposits now forming in the Mediterranean in deep water are all blue muds, with a yellowish upper layer, containing usually from 10 to 30 per cent. of carbonate of lime, which principally consists of the shells of pelagic Foraminifera. The mineral particles and clayey matter are derived from the disintegration of the neighbouring land. In some of the shallower depths there are glauconitic and more calcareous deposits. The deep-sea dredgings show that life, though present, is much less abundant in deep water than at similar depths in the open ocean, in which respect the Mediterranean agrees with enclosed seas in general. There is an extensive red coral fishery and tunny fishery on some parts of the coasts. The Mediterranean region appears to have been covered by the sea from early geological times, and during Tertiary times must have had much wider communication with the open ocean.