The hydrosphere describes the combined mass of water found on, under, and over the surface of a planet. The Earth's hydrosphere contains about 1.4 × 1018 tonnes of water, with 75% of the surface covered by oceans. The hydrosphere includes all liquid and frozen surface waters, groundwater, and atmospheric water vapor, which are constantly circulating through the hydrologic cycle. Water pollution occurs when pollutants from sources like sewage, chemicals, and oil are discharged directly or indirectly into bodies of water without adequate treatment. Common causes of water pollution include inadequate treatment of sewage, dumping of industrial chemicals and waste, and oil spills from transport ships and inadequate cleanup efforts.