An aquifer is an underground layer of porous rock, gravel, sand and/or silt that is saturated with groundwater. Water, enters aquifers from precipitation and other surface water that seeps through the soil, and it can move through the aquifer and then resurface through springs and be extracted with wells.
Aquifer depletion is a growing problem in much of the world and will eventually have serious consequences, particularly with regard to food production. An example is the vast Ogallala Aquifer, one of the world's largest and which underlies portions of eight states in the central United States. Its water is primarily fossil water from the time of the last glaciation, and its recharging rate is low because it is in an arid or semi-arid ecosystem in which rainfall is low and generally less than evaporation. In some locations it is being rapidly depleted by the continuing large-scale pumping for agricultural use and and also for household and other uses. The annual recharge in the more arid parts of the aquifer is estimated to total only about ten percent of annual withdrawals.