This photograph shows a location southwest of Riyadh in the Jabal Tuwayq area of Saudi Arabia. The circular features that dot the desert landscape in this near-vertical photograph are created by pivoting sprinklers, a method of irrigation whereby water from a central well is spread by a sprinkler pivoting around that well. Center-pivot irrigation is most often used in arid and semiarid regions of the world. The diameter of these circular fields can vary from several hundred feet (tens of meters) to over a mile (2 kilometers). This extensive area of center-pivot irrigation is one of four in Saudi Arabia that have been documented by recent Space Shuttle missions. The water is being extracted from subsurface reserves that infiltrated deep aquifers during geological periods when the climate was much wetter than at present. Center-pivot irrigation is not unique to Saudi Arabia, and there is danger in that there is a finite supply of fossil water for irrigation use.