The hydrosphere describes the collective mass of water found on, under, and over the surface of a planet. It includes all water in its various forms present in Earth's environment, including liquid water in oceans, lakes and underground; solid ice in ice sheets; water vapor in the atmosphere; and water in living things. The major parts of Earth's hydrosphere are the oceans, which contain an average of 1300 million cubic kilometers of water at depths around 3.7 kilometers, and ice sheets, which hold around 24 million cubic kilometers, mostly in Antarctica.