An aquifer is an underground layer of water bearing rock. Many areas on earth have an underlying aquifer, which can supply fresh water through the use of a well. Aquifers were first tapped for water in the early 1900's for irrigation. Aquifers were originally believed to be an inexhaustible resource thus humans began tapping them for municipal and industrial uses as well. Now, many countries are over pumping aquifers in a struggle to fulfill their growing water supply demands. For example, in the United States, the water levels of the Ogallala aquifer have been lowered 100 feet at a rate of 40 times faster than nature can refill it. While aquifers are critical to human habitation and farming, they can be and are being over exploited. There are several problems that can result from over exploitation of aquifers. Subsidence is a problem that occurs from groundwater extraction in unconsolidated aquifers. An unconsolidated aquifer is composed of porous gravel, sand, and silt. When to much water is removed from this kind of aquifer, the unconsolidated materials settle causing the ground to sink and the ability of the aquifer to replenish itself is permanently removed or reduced. Salt water intrusion is another problem that can occur when to much water is pumped from the ground. However, this only occurs in aquifers near the coast. Costal aquifers have salt water layers beneath the fresh water layers. When the fresh water layer is over pumped, salt water enters the fresh water and contaminates it. When this occurs, the water is no longer fit for human consumption. The ultimate problem is that over pumping can lead to exceeding the sustainable yield of the aquifer, meaning that more water is removed than can be replaced.