The hydrosphere is the mixed mass of water found on, under, and above the surface of a planet, minor planet or natural satellite.

It has about 1386 million cubic kilometers of water on Earth. All these millions cubic kilometers of water include water in liquid and frozen forms in groundwater, oceans, lakes and streams. 97.5% of all water is salt water. Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers. 30.8% is in the form of fresh groundwater. Only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems. The total mass of the Earth’s hydrosphere is about 1.4 × 1018tonnes, which is about 0.023% of Earth’s total mass. Earth’s atmosphere contains about 20 × 1012tonnes of this water. In practical purposes, 1 cubic meter is about one ton. Approximately 75% of Earth’s surface is of some 361 million square kilometers is covered by ocean. The average salinity of Earth’s oceans is about 35 grams of salt per kilogram of sea water (3.5%).

Water is a basic necessity of life. Since 2/3 of the Earth is covered by water, the Earth is also called the blue planet and the watery planet. So hydrosphere is a very important thing on the Earth. Oceans are important in this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of Mercury. Later the gases hydrogen and helium were expelled from the atmosphere. The gases and water vapor released as the Earth cooled became our present atmosphere. Other gases and water vapor released by volcanoes also entered the atmosphere.