What do we call the underground level below which all spaces in rocks are filled with water?

The term that describes the underground level below which all spaces in rocks are filled with water is the water table. It signifies the boundary between the unsaturated zone, where some spaces in soil and rock contain air, and the saturated zone, where all voids are filled with water. Understanding the water table is essential in hydrogeology because it helps determine how much groundwater is available for wells and influences various ecological and geological processes.

The other terms refer to related but distinct concepts. An aquifer is a geological formation that can transmit significant amounts of water, serving as a source of groundwater. A reservoir typically refers to a larger surface water body or a man-made structure designed to collect and store water. A spring is a natural discharge of groundwater, where water flows out of the ground, usually at a point where the water table intersects the surface. Therefore, while these terms are connected to groundwater, the water table is specifically the level that defines the saturated zone beneath the ground.