An underground layer of permeable rock, sediment or soil that can be tapped as a water source. Also known as a water table or water-bearing-stratum, an aquifer is made up of porous rocks with tiny holes–which is why it is able to store water. There are two types of aquifers: replenishable and non-replenishable fossil aquifers. Aquifer depletion is a growing environmental concern. Some countries are overpumping aquifers to meet growing water needs.