The hydrosphere is the combined mass of water found on under and above the surface of a planet or minor planet or natural satellite. Hydrosphere on Earth has been around for about four billion years and it continues to change in shape. This is caused by seafloor spreading and continental drift which rearranges the land and ocean. The frozen part of hydrosphere is made of ice. The frozen part of the hydrosphere has its own name the cryosphere. The water cycle refers to the transfer of water from one state or reservoir to another. Fresh water sources contain rivers or lakes and rain whereas saline water sources contain oceans and seas along with these two sources other reservoirs are groundwater and polar ice caps. Solar energy in the form of heat light and gravity cause the transfer from one state to another over periods from hours to thousands of years. Most evaporation comes from the oceans and is returned to Earth as snow or rain. Sublimation refers to evaporation from snow and ice. Transpiration refers to the expiration of water through the minute pores or stomata of trees. Marq de Villiers has described the hydrosphere as a closed system in which water exists. De Villiers claimed that on Earth the total amount of water has almost certainly not changed since geological times so what we had then we still have now. Water can be polluted and abused or misused but it is neither created nor destroyed it only migrates. There is no strong evidence that water vapour escapes into space. Water is a basic necessity of life.