While today the Sahara is a dry, sandy, mostly inhospitable place, this wasn’t always the case. Evidence from cave paintings made thousands of years ago show the region was once teeming with wild animals, from hippos to giraffes, and that ancient farmers even raised their cattle there. New evidence shows how this lush and verdant ecosystem may have been maintained, as researchers have discovered the remains of an ancient river system buried beneath the desert and lost to the sands of time.

Stretching for over 500 kilometers (300 miles) from the middle of the Sahara to the coast of Mauritania in West Africa, the river would have been fed by water from the Atlas Mountains to the north and Hoggar Mountains to the east. In fact, the river system was so vast that if it were still flowing today, it would be ranked as the 12^th largest drainage basin on Earth. The researchers, who have and have published their study in Nature Communications, believe that it once fed the proposed Tamanrasett River.

^{A map of the river systems and their basins associated with northern Africa, showing the ancient Tamanrasett River, top left. Skonieczny et al. 2015}

The possibility that a river system once existed in the region was first hinted at around a decade ago, following the discovery of fine river sediment and a deep underwater canyon carved into the continental shelf off the coast of Mauritania. However, direct evidence needed to confirm this was lacking. This time around, the scientists used orbital radar satellite imagery, which allowed them to take images of the geology of the Sahara meters below the sandy surface using microwaves. From this data, the scientists could see the ancient riverbeds of the waterway, which incredibly matched up with the canyon off the coast.

It’s estimated that the river has been periodically flowing during what are called the African humid periods (AHPs), the last of which ended around 5,000 years ago when the lush, wet and humid Sahara which teemed with animals and life, turned into the dry, dusty place we know today. These switches between the wet and dry periods are estimated to occur every 20,000 years or so, as the Earth wobbles on its axis. Whether or not the ancient river beneath the desert will flow again during the next AHP is difficult to determine, however, as climate change is currently disrupting weather patterns, and making things harder to predict.

This study goes to show how the climate can change incredibly rapidly, so fast that within just a few thousand years all surface evidence of this vast ancient waterway has been buried and the local ecosystem altered beyond recognition. The researchers plan on using the same system of satellites to help track down other ancient river systems concealed beneath the sands of the Sahara that could link to other underwater marine features.