It's the world's largest desert. It occupies most of northern Africa and is 3,000 miles (4,800 km) long and 800 miles wide.
As the desert extends and contracts, the area changes.
The highly crumpled and denuded Precambrian African Shield underlies the Sahara.
Paleozoic formations deposited after the shield's stability have remained horizontal and unmodified.
About 25% of the Sahara is covered with sand dunes and sheets.
Tied dunes, which grow in the lee of hills or other impediments; parabolic blowout dunes;
Several non-Saharan rivers feed the desert's surface and groundwater.