Water is one of the essential components necessary for life to exist on Earth. All of Earth's water in all its states, frozen, liquid, and gaseous, make up the hydrosphere. Liquid water exists on the surface of the earth but there are also reservoirs of water underground. Frozen water exists as glaciers, ice caps, and icebergs. This water makes up a special subcategory of the hydrosphere called the cryosphere. Water can even be found in the atmosphere in the form of water vapor. Readers will come to understand how the water cycle tracks water's movement from one phase to the next.