An aquifer is a geological formation that can store and transmit water. It is a permeable layer of rock, sand, gravel, or other material that contains water and allows it to move through the porous spaces. Aquifers are recharged by precipitation and other surface water, and are typically located underground. They are important sources of water for wells, springs, and other types of water supply systems, especially in areas with limited surface water resources. Aquifers can be susceptible to over-extraction and contamination, so it is important to manage their use sustainably.