Artesian Well, a well in which water rises under its own pressure, without pumping. If the pressure is great enough, the water will rise all the way to the surface and flow freely from the well. The name "artesian" is derived from Artois, France, where such wells were sunk as early as 1126.

An artesian well is drilled into an aquifer, a water-bearing layer of porous rock. The aquifer lies between two impervious layers of rockthat is, layers of rock through which water cannot pass. The artesian system (the aquifer and the impervious layers) slopes, and the well is sunk at a point where the aquifer is lower than the place where water, from rain or melting snow, enters the system. The weight of the water held in the upper portions of the aquifer results in the pressure that forces water to rise in the well. Where there is a natural opening in the impervious layer above an aquifer, water is sometimes forced to the surface, forming an artesian spring.

The water in many artesian systems is pure, while in others it is strongly mineralized. The principal areas where artesian systems occur in the United States are the Great Plains, from North Dakota to Texas, and the Atlantic Coastal Plain. The Central Valley of California is also an artesian area. Portions of the Sahara have been reclaimed by the use of artesian wells. Australia's Great Artesian Basin, a vast area to the west of the Great Dividing Range, is noted for its artesian wells.