Although groundwater exists everywhere under the ground, some parts of the saturated zone contain more water than others. An aquifer is an underground formation of permeable rock or loose material which can produce useful quantities of water when tapped by a well.
Aquifers come in all sizes.
They may be small, only a few hectares in area, or very large, underlying thousands of square kilometers of the earth's surface. They may be only a few meters thick, or they may measure hundreds of meters from top to bottom. It is a common misconception that groundwater is found in underground rivers like those that form limestone caverns. In fact, groundwater is more like the water in a sponge, held within the tiny pores of the surrounding aquifer material. Much like the flow of water in a river, however, the flow of groundwater is subject to gravity and is almost always in motion, flowing from areas of higher elevation to areas of lower elevation.