An aquifer is a water-bearing rock layer that can store and transmit groundwater. It is usually found between two layers of impermeable rock that prevent the water from seeping out. The permeable rock can be sandstone, limestone, or other types of rock that allow water to flow through their pores and spaces. Aquifers are important sources of water for wells and springs, and they can be replenished by precipitation and surface water that percolates down into the ground.