It’s one of the largest projects in the history of Africa: a 4,400-mile wall stretching from Senegal to Djibouti along the edge of the Sahel region, just south of the Sahara Desert. However, the gigantic barrier isn’t made of bricks and mortar but trees. And it’s not designed to keep people in or out, but to stem one of the most serious problems facing the north of the continent: desertification. The Sahara is constantly growing, gobbling up precious arable land that could be used for farming. Announced in 2005, the Great Green Wall is a nine-mile-wide strip of indigenous trees and shrubs that will, hopefully, retain groundwater, provide shade, be a food source for livestock, provide employment for the local population and even prevent extremism. “The Great Green Wall is about more than just planting and counting trees, it is about building resilience in communities and developing sustainable projects to give young people reasons to stay,” said Camila Nordheim-Larsen of the UN Convention to Combat Desertification (UNCCD). So far, the project’s progress has been slow but promising: as of March 2016, 3,746,777 trees had been planted in Burkina Faso and Senegal has reclaimed vast swathes of land (over four million hectares) for the scheme.