Dominating the central Sahara are the mountainous areas of the Ahaggar, in southern Algeria, and the Tibesti, which lies mainly in northern Chad. These rugged and barren highlands of volcanic origin have been eroded by wind and water into many strangely shaped peaks. The extinct volcano of Emi Koussi, in the Tibesti, reaching 11,204 feet (3,415 m), is the Sahara's highest point; Tahat, in the Ahaggar, rises to 9,573 feet (2,918 m). Southward lie two mountainous extensions of the Ahaggar, the Air and the Adrar des Iforas. The Air descends abruptly eastward to the flat, sandy plains of the Tenere in northern Niger.
The western Sahara is a low, gently sloping region of dunes and gravel plains interspersed with a few low hammadas and ranges of low hills. Its largest areas are the Tanezrouft, a featureless reg that is one of the most desolate and arid parts of the Sahara; the El Djouf desert; Erg Chech; and Erg Iguidi. Only a small portion of the western Sahara lies more than 1,000 feet (300 m) above sea level, and large parts of the far west are below 500 feet (150 m).
In the northern Sahara are the huge sand wastes of the Great Eastern and Great Western ergs. Near the base of the eastern Atlas Mountains, salt flats and intermittent salt lakes known as chottsoccupy basins that dip below sea level at points.
Most of the eastern Sahara is occupied by the Libyan Desert, a 500,000-square-mile (1,300,000-km2) region of dunes and rock outcrops. Along its northeastern margin, in Egypt, is the Qattara Depression, which descends 436 feet (133 m) below sea level— second lowest point on the continent.