A geological term meaning a water-bearing rock. Aquifers are important sources of water. Wells are sunk to aquifers and the water pumped to the surface.

A diagram showing how an aquifer works. If water is trapped underground in an aquifer it is called artesian water.

Water pouring through limestone in a cave. Most limestones are aquifers.

Many types of rock make good aquifers, but sandstone and chalk are the best known. Aquifers allow people to live and work even in places where there is little surface water such as deserts. One of the world's most extensive aquifers lies under much of Australia (The Great Australian Basin) and farmers tap into it to provide both irrigation water and drinking water for livestock. An immense aquifer (The Olgallala aquifer) also underlies the Great Plains region of the U.S., and this is heavily used to allow crop farming.

This headwater only flows when the level of water in the chalk below rises to the surface during the winter. Notice that there is no river channel; the water has simply risen up through the soil and grass.