In response to gravity, water seeps into the ground and moves downward until the rock is no longer permeable. The subsurface zone in which all openings of the rock are filled with water is called the zone of saturation. The upper surface of the zone of saturation is called the water table. The zone that exists between the water table and the ground surface is called the zone of aeration. In order to be successful, a well must be drilled into the zone of saturation. The velocity at which water flows underground depends on the permeability of the rock or how large and well connected the openings are.
Springs occur where water flows naturally from rock onto the surface of the land. Springs may seep from places where the water table intersects the land surface. Water may also flow out of the ground along fractures.