Mention theSahara Desert and many people think ofsand dunesstretching from horizon to horizon. While enormous fields of sand cover some parts of the Sahara, that sand only covers about 20% of the world's largest desert. The rest consists of rockyplateaus, open plains ofgravel, and barren rocks.

From space, the Sahara is amottledpatchwork of browns and yellows. The bright yellow-white sand fields are especially prominent in the eastern and western regions, while the wrinkled, brown Ahaggar and Tibesti mountain ranges dominate the central area.

In the west, the great El Djouf basin ("The Belly") holds one of the largest expanses of sand in the world. This almost completely lifeless "empty quarter" ofMauritaniais more than 1000 kilometers (600 miles) across. In the east there is even more sand, mostly concentrated in windblown trails running southwest across Egypt and Libya.