Although ground water is an important source of fresh, liquid water, it does not always occur where it is most needed and sometimes it is difficult to extract. Ground water is perceived by some to occur as vast underground lakes and rivers, but generally, ground water occurs in voids between soil or rock particles in an area called the zone of saturation.
Ground water occurs in many different types of soil and rock strata.
The amount of ground water that is stored and released from water-bearing strata depends on the porosity, size of pore spaces, and continuity of pores. Water-bearing porous soil or rock strata that yield significant amounts of water to wells are called aquifers. An aquifer can range from a few feet to hundreds of feet thick and can underlie a few acres or thousands of acres. For example, the Ogallala aquifer extends from Texas to South Dakota in the United States. Water flowing through a soil or rock material follows the path of least resistance.