Building a wall of trees across the width of Africa is a tall order. Solving the twin problems of land degradation and desertification poses a greater challenge still. But more than 60 years after it was first proposed, just such a project is underway at the edge of the Sahara. The opening ceremony of the Olympics in Rio de Janeiro thrust it into the limelight, with footage of its progress. On completion, it is set to be the world’s largest living structure, three times the size of the Great Barrier Reef. What is Africa’s “Great Green Wall”?