Groundwater is water under the Earth’s surface. Most groundwater is found in porous rocks, which have tiny holes in them. If a hole is bored straight down through the rock, groundwater is eventually found at a certain level. This level is called the water table, and it usually rises when rainwater soaks into the ground. A spring is a place where groundwater emerges from a hillside.

An aquifer is a layer of porous rock that can fill with water, like an underground reservoir. Sometimes, part of an aquifer is covered by rock that water cannot flow through, such as clay. This forms an Artesian basin. If a well is sunk through the basin to the aquifer, water flows into the well.