Sahara, the largest desert in the world, occupying an area estimated at from 1,500,000 to 2,500,000 sq. m., in the N. portion of Africa, across which it extends 3,000 m. from the Atlantic ocean to the valley of the Nile, with a width of 1,000 m. between Soodan and the countries bordering the Mediterranean sea. As the same sterile region is renewed beyond the Nile, its E. and W. boundaries may be regarded as coterminous with those of the continent itself, while the 15th and 30th parallels of N. latitude mainly form its southern and northern limits. Under the name of the Algerian Sahara, it extends considerably N. of the 30th parallel along the southern base of the Atlas, and closely approaches the Mediterranean W. of the gulf of Cabes. Here the marshy depressions known as shotts, of which the so-called Melrir lake is best known, constitute a basin into which it has been proposed to admit the waters of the sea by means of a canal. Continuations of the Sahara stretch eastward and northward through Arabia, Persia, and central Asia into Mongolia, where they terminate in the desert of Gobi. The great desert of Africa presents an alternation of immense burning wastes of loose and moving sand, with tracts of barren rock, stony plains of gravel, many of which are covered with saline deposits, and elevated and rocky plateaus rising into mountains with extensive valleys and expanses of. sand between them.

The average elevation of the Sahara above the sea is estimated at 1,500 ft., although the surface is known to be depressed in many places below the level of the ocean. Its most mountainous portion lies along the caravan routes between Tripoli and Bornoo and Houssa, where the general altitude of the table land, beginning on the north in the plateau of Hamadah, ranges from 1,000 to 2,000 ft., amid peaks of double these heights, attaining the maximum in the mountains of the oases of Asben, 5,000 ft. The desolate region of the west, called Sahel (the plain), comprises the greatest expanse of sand and salt desert. Its hills encroach upon the Atlantic in a long line of banks pushing out into the sea between Cape Bojador and Cape Blanco. In this portion of the desert oases are few and small, and there is little travel. East of the oasis of Fezzan the Sahara becomes known as the Libyan desert, and is comparatively level, sloping toward the Mediterranean with a gentle gradient. (See Libyan Desert.) Here oases, the only permanently inhabited places of the desert, are most numerous. (See Oasis.) The character of the S. border of the Sahara is imperfectly known, but is believed to be mountainous.

In the north it is skirted by extensive treeless pasture lands along the base of the Atlas range, reaches the shores of the Mediterranean at the gulf of Sidra, and further E. is bounded by the table land of Barca. - A climate of burning aridity prevails on the great desert of Africa. Rain is utterly unknown except in the oases and on the mountains, where it occasionally falls with such violence as to produce torrents that suddenly pour down into the valleys, and almost as suddenly disappear. The sterility of the Sahara is largely attributable to the fact that the prevailing N. E. trade winds which blow over its surface bring it no moisture, having been almost drained of aqueous vapor in their long continental journey over Europe and Asia. On the mountains S. of the Mediterranean they deposit more than they have collected in their brief passage over that sea. When they reach the heated desert beyond, where the absorptive capacity of the air is greatly increased by the access of temperature, they bear away moisture instead of bringing it, and it is not condensed into rain until it reaches the mountains of central Africa. The Sahara is probably subject to a higher temperature than any other region on the globe; the thermometer there has been known to register 133° F. This terrific heat imparts their dreaded characteristics to the simoom and other similar winds which blow off the desert. (See Africa.) Notwithstanding the obstacles to travel offered by the desert, it is constantly crossed on various routes by caravans of traders.

In the absence of watercourses, were it not for the camel, well termed in oriental language "the ship of the desert," these wastes would be impassable to man. Nocturnal radiation is extremely rapid, and the nights are usually cold. - The geological formation is unfavorable to fertility, and although in some localities a growth of thorny bushes and plants peculiar to the desert is met with, the vast bodies of silicious sand afford little or no nutriment to vegetation, and their unstable nature is opposed to its development. Even in mineral productions the desert maintains its character as a barren waste. No useful products are obtained except salt, which is collected by caravans and sold in Soodan. The fauna of the desert proper includes snakes, lizards, scorpions, and ants; but the animals of its borders and the oases comprise about 50 species of mammals, including the lion and leopard, the giraffe, and various antelopes. The marked distinction between the faunas of N. and S. Africa, separated by the desert, is explained by the supposition that a portion of the Sahara was submerged beneath the sea during the pliocene period of geology.

Marine shells have been discovered S. of the Atlas, and lines of sea beach, showing that at no very remote geological period the plains formed the bed of the ocean. According to Sir Charles Lyell, the Sahara was under water between lat. 20° and 30° N. at one time during the glacial epoch, so that there was water communication between the southern part of the Mediterranean and that portion of the Atlantic ocean now bounded by the W. coast of Africa. A project has of late been advocated of reconverting the deeper part of the Sahel, an area of about 126,000 sq. m., into a sea, by cutting a canal from the Atlantic through the sand hills which form the western border.