This interesting History Channel documentary explains that the Sahara Desert has been a variety of radically different types of landscapes over the millennia, including being a vast sea 40 million years ago, and it also explains how the area currently shifts between being a desert and a lush grassland every 20 thousand years.

Following are points this documentary makes:

— The Sahara Desert was once a vast swampy sea 40 million years ago, which has caused the desert to be full of fossils of marine life, including from large whales.

— Core samples from the ocean floor show that the Sahara has been a desert for three million years, and the same core samples show that the desert oscillates in a cyclical manner between between being a desert and being a lush grassland every 20 thousand years due to a wobble in the Earth’s rotation.  [NOTE: I think it is great that this documentary acknowledges the naturally cyclical nature of climate change.]

— Radar scans made from space in 1981 show a hidden network of dried ancient waterways criss-crossing the desert under the sand.

— Large amounts of fossils of fresh-water marine life show that the desert has previously received a lot of rainfall and it had gigantic lakes that were three times larger than the Great Lakes in the United States.  Stone tools can be found where the shores of those lakes had existed, along with fossil remains of animals that people had hunted.  The lakes formed a “green corridor” through the center of the Sahara, where descendants of humans who migrated to the northern continents would have easily been able to pass through.

— Much evidence of human habitation has been found throughout the Sahara, being carbon dated show that the desert was a lush grassland only seven thousand years ago.

— The ocean cores show that it only takes a few hundred years for the desert to revert back to being a grassland once the shift in climate happens.

— The Sahara desert has massive amounts of anicent water underneath it in gigantic aquifers, which is more evidence that the region was previously very wet.