An artesian well is a deep well through which water is forced upward. The drill hits an aquifer, which is an underground water source. Rain water flows underground to this source. In South Dakota, the story of the artesian wells starts in the rocks of the Black Hills, which help the water flow into the ground. Underground the water spreads across the state. The first wells drilled into this source gushed up like oil wells. 

Artesian Well
Photo courtesy of South Dakota State Historical Society