Overview
Not all water on Earth exists in lakes, rivers, and seas. Some water can be found underneath the surface of the Earth as ground water. Ground water is an important source of water for plants and irrigation.
The Water Table
Most soil, sediment, and sedimentary rock such as sandstone, is porous, so that rain falling on it soaks into the ground and fills open spaces. Water seeps down between those spaces to collect into an area where the ground is completely saturated and very moist. If a deep hole is dug below the water table, the bottom of the hole or well will fill with water, even when no rain has fallen recently. The water table is at the top of the zone of saturation, and its depth may vary with seasonal rainfall or with the type of material surrounding the zone of saturation.
Ground Water
Most ground water seeps toward rivers and streams, so that water flows in stream beds and feeds them, even during times of no rainfall. If ground water bubbles up to the surface, it is called a spring. Often, ground water is available for use in irrigation when it is drawn from wells that are drilled below the water table.
Sinkholes
If water is drawn up from wells at a greater rate than it can be replenished, the land around the aquifer often becomes unstable and collapses. For example, the land under the San Joaquin Valley in California sunk in several places, possibly from water drawn to irrigate crops that are grown there. If the roof of a limestone cavern collapses, sinkholes can create destruction. Everything on the surface, including houses, falls into the sinkhole and disappears from view.
Hot Springs and Geysers
When ground water is heated by volcanic activity below the surface, resulting springs are heated. Most of the time, hot water bubbles or flows from cracks in underlying rock, but sometimes geysers erupt from intense pressure. Bubbles of water vapor are trapped within vents and explode. Many volcanic areas in Yellowstone National Park in North America, as well as areas in Iceland, feature hot springs and geysers as a result of heated ground water.
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