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.

Why are the agricultural plots perfectly round? The oasis relies on center-pivot irrigation, in which a single, rotating beam of sprinklers delivers water to crops planted around a central point. The size of the plots—up to one kilometer in diameter—helps them stand out in this image from Japan's Advanced Land Observing Satellite.