Groundwater is water stored underground in aquifers, which are geological formations that can hold and transmit water. There are three main types of aquifers: crystalline, consolidated, and unconsolidated. Water in aquifers flows from recharge areas to discharge points like springs or rivers. The movement of groundwater is driven by the hydraulic gradient, which is the difference in water level between recharge and discharge areas divided by the distance water travels. The flow of water through aquifers can be described by the Darcy equation, which is based on experiments conducted in the 1850s.