United StatesGroundwater 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.
Even in arid (dry) places such as deserts, groundwater sometimes comes to the surface. These lush green areas are called oases. The water at an oasis may have travelled underground from mountains hundreds of kilometres away. Oases are an important source of water, and towns often grow up around them.
