The name “Sahara” is derived from the Arabic word for “desert” in the feminine irregular form, the singular ṣaḥra The desert comprises much of North Africa. The Sahara can be divided into several regions, including the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, and the Libyan Desert. For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 20,000 year cycle. Sand dunes and sheets cover only around 25% of the Sahara’s actual surface. Despite its large size, only around 2.5 million people live in the Sahara Desert.