About 20% of the Earth is a desert. Deserts are places that get very little precipitation (rain or snow) each year, and that makes them extremely dry. Deserts cover big areas of land. The biggest desert, the Sahara, extends from North Africa to Southwest Asia and is 13 times the size of Texas. Some parts of the Sahara get as little as 2 millimeters (0.08 inches) of water a year.