The hydrologic cycle, or water cycle, circulates water through our planet and its atmosphere. The whole cycle is powered by the sun. Water evaporates from the world's oceans and lakes, from the leaves of trees and even from the exhalations of animals and people. This water vapor rises in the atmosphere and becomes clouds. As it cools, it condenses into drops that fall as rain, snow, sleet or hail, returning to Earth to start the cycle again. However, there are large reserves of water that spend long periods of time stored underground. The oldest of this water is often referred to as "fossil water."
Of course, water can't fossilize in the same way that bones or other objects can, but stores of groundwater that have stayed in underground aquifers for millions of years seem to have earned the title. In the United States, the oldest water can be found in the Ogallala Aquifer, which holds at least 2.9 billion acre-feet (3.6 billion kilometers cubed) of water [source: Kansas Geological Survey]. This huge aquifer lies underneath eight of the Great Plains states; it's believed that the underground reservoir was formed alongside the creation of the Rocky Mountains, which would make it between 2 and 6 million years old.
When we think of mining, we usually picture rocks, coal or maybe diamonds dug out of the ground. The term "mining," meaning the act of taking non-renewable resources out of the ground, has been borrowed to refer to the exploitation of fossil water resources. Little, if any, new water enters fossil water aquifers like Ogallala, which is why fossil water is considered a non-renewable resource. Some scientists also talk about mining for water on other planets or moons in the future to supplement Earth's dwindling resources and to help support space colonies.
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