The hydrologic cycle is the continuous circulation of water throughout Earth and between Earth's systems. At various stages, water—which in most cases is synonymous with the hydrosphere—moves through the atmosphere, the biosphere, and the geosphere, in each case performing functions essential to the survival of the planet and its life-forms. Thus, over time, water evaporates from the oceans; then falls as precipitation; is absorbed by the land; and, after some period of time, makes its way back to the oceans to begin the cycle again. The total amount of water on Earth has not changed in many billions of years, though the distribution of water does. The water that we see, though vital to humans and other living things, makes up only about 0.0001% of the total volume of water on Earth; far more is underground and in other compartments of the environment.

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