 Hydrosphere, the hydrosphere is the combined mass of water found on, under, and above the surface of a planet, a minor planet or natural satellite. Although the hydrosphere has been around for longer than for billion years, it continues to change in size. This is caused by the seafloor spreading and the continental drift, which rearranges the land and ocean. It has been estimated that there are 1,386 million cubic kilometers of water on Earth. This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount. Heat is another form water is in, caused by the rapid increase in particles transferred by heat. Fresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form of ice and plain snow covered 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 1.41018 tons, which is about 0.0 to 3% of Earth's total mass. About 2010-12 tons of this is in Earth's atmosphere for practical purposes. One cubic meter of water weighs one ton. Approximately 71% of Earth's surface, in area of some 361 million square kilometers 139.5 million square miles is covered by ocean. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water 3.5%.