An aquifer is a layer of water contained within underground rock, sand, gravel, or clay. Although the term does not necessarily imply any particular size, very large freshwater aquifers are especially important because they can be accessed via wells and provide important sources of drinking water for large numbers of people.
Aquifers can be of any size and occur at any depth below the surface, although, obviously, those which are larger and located in shallower ground can be more easily accessed through human technology. There is a vast amount of water located underneath the surface of the Earth (although most water on Earth is contained in the oceans). In certain rock formations, this actually takes the form of underground rivers. In more cases, it involves a region of porous rock which has been saturated, or had these empty spaces filled, with water, much like a giant rocky sponge.
Particularly in areas with minimal river coverage (such as arid areas), aquifers can be a rich and critical water resource for human populations, accessed via large numbers of wells and allowing for drinking water, irrigation water, and other possibilities where otherwise human habitation in large numbers would either be impossible or would need to rely on importing water from elsewhere.
Intensive use of aquifers, however, can come with serious consequences. Although vast, even large aquifers are not unlimited resources, and replenish only quite slowly over time. Over-exploitation through wells can therefore deplete aquifers, threatening long-term water availability for what becomes, in many cases, substantial populations. Along the coastline, this threat becomes all the more serious because a lowered water table may top itself up with saltwater from the nearby ocean, thus contaminating the water supply with salt. There are some notable exceptions, such as the large Edwards Aquifer in Texas, which recovers very quickly by taking water from nearby rivers and streams, and therefore is still full despite intensive human use.
Other aquifers, however, are not so fortunate, including some of the world's great aquifers. One of the best-known aquifers is the Ogalalla Aquifer, a vast formation lying underneath parts of eight states on the Great Plains. Most of the people living near the aquifer drink its water, and it also supplies about one-third of American irrigation water. Tens of millions of acre-feet of water are withdrawn from the aquifer every year for human consumption, an amount which far outstrips the aquifer's marginal capacity to recharge. Some places, for example in Texas, have already lost the capacity to reach useable levels of the aquifer, and pessimistic projections suggest that its remaining few billion acre-feet of water will be exhausted in the next century, barring significant measures to conserve water and reduce the amount withdrawn for irrigation.