The largest hot desert on Earth — a swath of sand, rock, and gravel covering roughly nine million square kilometers across northern Africa — that has split a continent, was once green savanna within the span of human habitation, and continues to shape the world far beyond its sands: driving global climate, fertilizing distant ecosystems, and marking the route that thousands of people now risk everything to cross.
The rock walls of the Tassili plateau in Algeria hold thousands of painted cattle. The cattle lived there around seven thousand years ago. So did hippos and giraffes, crocodiles and swimmers. The Sahara was a green, watered landscape. Then the rains moved south and the desert came. The largest hot desert on Earth is not a permanent fact. It is a climate event still unfolding.