text: sahara\n\nSahara (suhâr'u) [key][Arab.,=desert], world's largest desert, c.3,500,000 sq mi (9,065,000 sq km), N Africa; the western part of a great arid zone that continues into SW Asia. Extending more than 3,000 mi (4,830 km), from the Atlantic Ocean to the Red Sea, the Sahara is bounded on the N by the Atlas Mts., steppelands, and the Mediterranean Sea; it stretches south c.1,200 mi (1,930 km) to the Sahel, a steppe in W and central Africa that forms its southern border. The desert includes most of Western Sahara, Mauritania, Algeria, Niger, Libya, and Egypt; the southern portions of Morocco and Tunisia; and the northern portions of Senegal, Mali, Chad, and Sudan. The E Sahara is usually divided into three regions—the Libyan Desert, which extends west from the Nile valley through W Egypt and E Libya; the Arabian Desert, or Eastern Desert, which lies between the Nile valley and the Red Sea in Egypt; and the Nubian Desert, which is in NE Sudan.\n\nRegions of sand dunes (erg) occupy only about 15% of the Sahara; “stone deserts,” consisting of plateaus of denuded rock (hammada) or areas of coarse gravel (reg), cover about 70% of the region; mountains, oases, and transition zones account for the remainder. Sparse vegetation is found in most parts of the Sahara, with the exception of the sand dune regions. High mountain massifs rise in the central regions; they are the Ahagger (Hoggar) in S Algeria, which rises to more than 9,000 ft (2,740 m); the Tibesti Massif in N Chad, which rises to more than 11,000 ft (3,350 m); and the Aïr Mountains (Azbine) in N Niger, which rise to more than 6,000 ft (1,830 m). The mountains are deeply dissected and were in the past infamous for the shelter they provided to marauders preying on desert traffic. From west to east the four principal land routes across the desert are from Colomb-Bechar to Dakar; from Colomb-Bechar to Gao and Timbuktu by way of Reganne; from Touggourt to Agadez and Kano by way of In-Salah; and from Tripoli to Ghat.