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The Ounianga Lakes in northeastern Chad are featured in this image from the International Space Station. This view features one of the largest of a series of ten, mostly fresh water lakes in the Ounianga basin in the heart of the Sahara Desert. The lakes are the remnant of a single large lake – probably tens of kilometers long – that once occupied this remote area approximately 14,800 to 5,500 years ago. As the climate dried out during the subsequent millennia, the lake was reduced in size and large wind-driven sand dunes invaded the original depression dividing it into several smaller basins. The area shown in this image measures approximately 11 x 9 km, with the dark water surfaces of the lake segregated almost completely by orange linear sand dunes that stream into the depression from the northeast. The almost year-round northeast winds and cloudless skies make for very high evaporation (an evaporation rate of greater than 6 m per year has been measured in one of the nearby lakes). Nonetheless, only one of the ten lakes is saline because a very large aquifer reaches the surface in the Ounianga depression in the form of the lakes. The aquifer is large enough to keep supplying the small lakes with water despite the high evaporation rate. Mats of floating reeds also reduce the evaporation in places. The lakes form a hydrological system that is unique in the Sahara Desert. Image courtesy of NASA/Jeffrey Williams.