For a long time, the Sahara was considered an expanse of sand and rock, but in the 1950s, a researcher and his guides discovered hundreds of thousands of cave paintings in southern Algeria. These masterpieces, dating back to prehistory, reveal an unexpected past. Before being a desert, the Sahara was a verdant El Dorado where lions and ostriches were hunted, and where complex cultural societies developed. A vast archaeological investigation is shedding light on a little-known chapter in human history: the metamorphosis of the Sahara. Using the latest investigative technologies, scientists are discovering new evidence buried beneath the sand, submerged at the bottom of the ocean, or etched in stone.