Outside Al Jawf, Libya, a verdant oasis blooms in the middle of a desert. Farmers irrigate their Saharan fields, which receive only 2.5 millimeters of rain a year, with so-called fossil water from a massive aquifer beneath a large swath of northeastern Africa. The Nubian Sandstone Aquifer System is a remnant of wetter eras going back 20,000 years, when heavy rains fell that ultimately penetrated more than three kilometers into the earth.