The Sahara is the largest hot desert in the world and the third-largest desert overall, covering 9.2 million square kilometers—roughly the size of the United States. It spans nearly the entire northern third of the African continent. The landscape varies from high sand dunes (ergs) to stone plateaus (hamadas) and dry valleys (wadis). Recent geological studies have shown that the Sahara cycles between being a desert and a lush savanna every 20,000 years due to changes in the Earth's axis. Currently, it is in a dry phase and is expanding southward into the Sahel region, a process called desertification. To combat this, several African nations have launched the 'Great Green Wall' initiative, planting a belt of trees across the continent to stop the desert's growth.