The document discusses the hydrosphere, which refers to the combined mass of water found on, under, and over the surface of a planet. It notes that the Earth's hydrosphere totals about 1.4 × 1018 tonnes, with about 20 × 1012 tonnes located in the atmosphere. Approximately 75% of the Earth's surface is covered by oceans, which have an average salinity of 35 grams of salt per kilogram of sea water. The hydrological cycle involves evaporation from surfaces like oceans and transpiration from plants, which adds water vapor to the atmosphere. Water pollution is a major global problem and can be caused by industrial discharges, agricultural and construction runoff, improper waste disposal, and other