An underground formation that contains groundwater is called an aquifer. Groundwater can dissolve rock formations and fill vast caves with water which creates underground lakes.
Aquifers are underground layers of rock that are saturated with water that can be brought to the surface through natural springs or by pumping.
The groundwater contained in aquifers is one of the most important sources of water on Earth: About 30 percent of our liquid freshwater is groundwater, according to the National Oceanic and Atmospheric Administration (NOAA). The rest is found at the surface in streams, lakes, rivers and wetlands.
Most of the world's freshwater — about 69 percent — is locked away in glaciers and ice caps. The U.S. Geological Survey website has a map of important aquifers in the contiguous United States.
Groundwater can be found in a range of different types of rock, but the most productive aquifers are found in porous, permeable rock such as sandstone, or the open cavities and caves of limestone aquifers.
Groundwater moves more readily through these materials, which allows for faster pumping and other methods of extracting the water. Aquifers can also be found in regions where the rock is made of denser material — such as granite or basalt — if that rock has cracks and fractures.
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