By Steve Graham, Claire Parkinson, and Mous Chahine Introduction
As seen from space, one of the most unique features of our home planet is the water, in both liquid and frozen forms, that covers approximately 75% of the Earth's surface. Believed to have initially arrived on the surface through the emissions of ancient volcanoes, geologic evidence suggests that large amounts of water have likely flowed on Earth for the past 3.8 billion years, most of its existence. As a vital substance that sets the Earth apart from the rest of the planets in our solar system, water is a necessary ingredient for the development and nourishment of life. 
Earth from space. (Image courtesy of NASA Jet Propulsion Laboratory.)
Water, Water, Everywhere
Water is everywhere on Earth and is the only known substance that can naturally exist as a gas, liquid, and solid within the relatively small range of air temperatures and pressures found at the Earth's surface. In all, the Earth's water content is about 1.39 billion cubic kilometers (331 million cubic miles) and the vast bulk of it, about 96.5%, is in the global oceans. Approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Finally, a thousandth of 1% exists as water vapor in the Earth's atmosphere. One estimate of global water distribution:
| Oceans, Seas, & Bays | 1,338,000 | 96.5 | - | | Ice caps, Glaciers, & Permanent Snow | 24,064 | 1.74 | 68.7 | | Groundwater | 23,400 | 1.7 | - | | Fresh | (10,530) | (0.76) | 30.1 | | Saline | (12,870) | (0.94) | - | | Soil Moisture | 16.5 | 0.001 | 0.05 | | Ground Ice & Permafrost | 300 | 0.022 | 0.86 | | Lakes | 176.4 | 0.013 | - | | Fresh | (91.0) | (0.007) | .26 | | Saline | (85.4) | (0.006) | - | Atmosphere
| 12.9 | 0.001 | 0.04 | | Swamp Water | 11.47 | 0.0008 | 0.03 | | Rivers | 2.12 | 0.0002 | 0.006 | | Biological Water | 1.12 | 0.0001 | 0.003 | | Total | 1,385,984 | 100.0 | 100.0 | Estimates of groundwater are particularly difficult and vary widely amongst sources, with the value in this table being near the high end of the range. Using the values in this table, groundwater constitutes approximately 30% of fresh water, whereas ice (including ice caps, glaciers, permanent snow, ground ice, and permafrost) constitute approximately 70% of fresh water. With other estimates, groundwater is sometimes listed as 22% and ice as 78% of fresh water. next: A Multi-Phased Journey
| |  The Water Cycle Introduction
A Multi-Phased Journey
The Water Cycle and Climate Change
The Aqua Mission and the Water Cycle
The Aqua Spacecraft
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