Whether constructed to manipulate societies or assert order over natural environments, humans have long loved walls. As literal barriers between areas, walls have the power to either stave off powerful forces or confine them. In the Sahel, a semi-arid region between the Sahara Desert of North Africa and the savannas of West Africa, international experts, national leaders, and local officials have conceived of a wall to keep the desert out. With modern crises like desertification and land degradation calling for a grand solution, the idea of a Great Green Wall, a 10-mile-wide swathe of trees designed to stretch more than 4,350 miles across the continent, is one of the grandest ever considered.