The Nubian Sandstone Aquifer System (NSAS) is the world's largest known fossil water aquifer system. It is located underground in the Eastern end of the Sahara Desert and spans the political boundaries of four countries in north-eastern Africa.

NSAS covers a land area spanning just over two million km2, including north-western Sudan, north-eastern Chad, south-eastern Libya, and most of Egypt. Containing an estimated 150,000 km3 of groundwater, the significance of the NSAS as a potential water resource for future development programs in these countries is extraordinary. 

The aquifer is largely composed of hard ferruginous sandstone with great shale and clay intercalation, having a thickness that ranges between 140–230 meters. Groundwater type varies from fresh to slightly brackish (salinity ranges from 240–1300 ppm).  

The Great Man-made River Project (GMMR) in Libya makes use of the system, extracting substantial amounts of water from this aquifer, removing an estimated 2.4 km3 of fresh water for consumption and agriculture per year. This system is primarily used to supply water in the Kufra oasis.

According to en.wikipedia