An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth’s surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 metres deep. Average oceanic salinity is around 35 parts per thousand (‰) , and nearly all seawater has a salinity in the range of 30 to 38 ‰. Scientists estimate that 230,000 marine species are currently known, but the total could be up to 10 times that number.