The hydrosphere is the collective mass of water at or near Earth’s surface. Beneath the land surface is the underground water component of the hydrosphere. The total amount of underground water is about 2.5 times greater than that contained in lakes and streams. Moreover, underground water is much more widely distributed than surface water. Whereas lakes and rivers are found only in restricted locations, underground water is almost ubiquitous, occurring beneath the land surface throughout the world. Its quantity is limited in certain areas, its quality is sometimes poor, and its occurrence is sometimes at great depth, but almost anywhere on Earth one can dig deep enough and find water. More than half of the world’s underground water is found within a half mile of the surface. Below that depth the amount of water generally decreases gradually. Although water has been found as deep as six miles underground, at that depth it is almost immobilized because the pressure exerted by overlying rocks is great and openings are few and small.

Almost all underground water comes originally from above. Its source is rain that either percolates ( trickles through) directly into the soil or seeps downward eventually from lakes and streams. Once the moisture gets underground, any one of several things can happen to it, depending largely on the nature of the soil and rocks it infiltrates. The quantity of water that can be held in subsurface material ( rock or soil ) depends on the porosity of the material, which is the total volume of the material that consists of pores or cracks that can fill with water. The more porous a material is, the greater the amount of open space it contains and the more water it can hold.

Porosity is not the only factor affecting underground water flow. If water is to move through rock or soil, the pores must be connected to one another and be large enough for the water to move through them. The ability to transmit underground water ( as opposed to just holding it, as in the definition of porosity ) is termed permeability, and this property of subsurface matter is determined by the size of the pores and by their degree of interconnectedness. The water moves by twisting and turning through these small, interconnected openings. The smaller and less connected the pore spaces, the less permeable the material and the slower the water moves.

The rate at which water seeps underground depends on both porosity and permeability. For example, clay usually has high porosity because it has a great many openings among the minute flakes that make up the clay, but it generally has low permeability because the openings are so tiny that the force of molecular attraction binds the water to the clay flakes and holds it in place. Thus, clay typically is very porous but relatively impermeable, so it can trap large amounts of water and keep it from draining.

Underground water is stored in, and moves slowly through, moderately to highly permeable rocks called aquifers ( from the Latin aqua, water, and ferry, to bear). The rate of movement of water varies with the situation. In some aquifers the flow rate is only a few inches a day; in others, it may be several hundred feet per day. A rapid rate of flow would be 40 to 50 feet per day. In contrast, water can move very slowly or not at all through certain materials. Impermeable materials made up of such elements as clay or very dense rock, which hinder water movement, are called aquicludes.

The general distribution of underground water can probably be best understood by visualizing the layers in a vertical subsurface cross section with at least three and often four hydrologic zones arranged one below another. From top to bottom, these layers are called the zone of aeration, the zone of saturation, the zone of confined water, and the waterless zone.