Picture the Sahara: 9 million km² (3.5 million square miles) of sand, the largest hot desert on Earth. Now rewind a sliver of geological time, and it disappears.
Around 6,000 years ago, much of the Sahara was green — grassland and woodland threaded with rivers and dotted with lakes. Giraffes, elephants, rhinos, crocodiles and hippos lived where there is now only dune. So did people: herding cattle, fishing, and painting the animals around them onto rock walls that still stand in the desert today.
This wasn't a one-off. Scientists call it the African Humid Period, and lake and ocean sediments record it happening more than 230 times over the past eight million years, on a roughly 21,000-year rhythm set by a slow wobble in Earth's orbit. The most recent green phase peaked about 6,000 years ago. Then the monsoon retreated, and in the southern Sahara the land flipped from lake to dune in just a few centuries.
The water is the part that's hard to picture. At its height, Palaeolake Mega-Chad covered roughly 360,000 km² (139,000 square miles) — bigger than the Caspian Sea, the largest lake on the planet today. Its shrunken descendant, modern Lake Chad, is down to a few hundred square kilometres.
For comparison — the desert is younger than you think:
- Green Sahara savanna: ~6,000 years ago
- Sahara dried into desert: ~5,500 years ago
- Great Pyramid of Giza built: ~4,500 years ago
- Lake Mega-Chad then vs Lake Chad now: ~360,000 km² shrunk to a few hundred km²
The sand we treat as timeless is barely older than civilisation. When the first cities were rising, the Sahara had only just begun to dry.
The hippos are still there — painted on rock walls, surrounded now by nothing but sand.