Geography
The Sahara was a green savanna with lakes and hippos a few thousand years ago
Between roughly 11,000 and 5,000 years ago, a regular wobble in Earth's orbit strengthened the African monsoon and covered today's Sahara in grassland, lakes and rivers rich enough to support hippos, giraffes and human settlements, recorded in rock art across the desert. The shift back to arid desert followed the same orbital cycle in reverse, in some regions surprisingly abruptly.