An aquifer is an underground layer of water-bearing permeable rock, sand, or gravel. (see also groundwater). The upper boundary of the topmost (open) aquifer is known as the water table. Some areas have several aquifers, each capped on top by an impervious layer (confined aquifer). If an aquifer is recharged, and the recharge area has a higher elevation than the capping layer, the groundwater in the confined aquifer may be under considerable pressure, and flowing or artesian wells are possible. Most land areas on Earth have some form of aquifer underlying them, though often at significant depth. Aquifers do not necessarily contain fresh water. Fresh water aquifers, especially those that do not recharge with groundwater, can be over-produced and draw-in non-potable water or saltwater. This can be a serious problem especially in coastal areas and other areas were aquifer pumping is high.
Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers.
Some aquifers are riparian aquifers. These are related to rivers, fluvial deposits or unconsolidated deposits of sand and gravel along river corridors, and are usually rapidly replenished.
Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface and have some percentage of recharge of freshwater. This recharge is typically from rivers or floodwaters that "leak" into the aquifer down fault zones, sink holes or through gravel deposits. The more substantial aquifers are in either carbonate rock or sandstone rock layers and have taken many millions of years to form. The less recharge an aquifer has the faster it will deplete.
An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer  in central Texas. This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas.
Aquifer depletion is a global problem, and is especially critical in northern Africa. However, new methods of groundwater management such as artificial recharge and injection of flood waters has extended the life of many freshwater aquifers, especially in the United States.
The Ogallala Aquifer of the central United States is one of the world's great aquifers, but is being rapidly depleted, primarily for agriculture use. This huge aquifer, present in around eight states, comprises fossil water from the time of the last glaciation. Recharge is minimal.
Excessive groundwater extraction may also lead to subsidence.