An ocean is a vast stretch of water which is renewed constantly by the marine currents. About three quarters of the Earth surface is covered by the Planetary Ocean, divided into five oceans and several dozen seas.

Way back in 1740, a Venetian naturalist called Moro suggested that seas and oceans salt content must come from deep within the planet - and brought to the surface by volcanoes. At the end of the 18th century, a huge controversy arose among the best known scientists of the time. Some of them (the Neptunian), claimed that all the ocean rocks were once made from sediments, while others (the Plutonian), tried to prove that the ocean rocks and water came from the Earth’s underground. Today, we know that water is the result of “condensing water vapors coming from volcanism”. Some water molecules might descend again into the deep crust of the earth and then they get recycled into the hydrosphere through volcanic eruptions.

The history of sea water salinity is very complex. Current sedimentary deposits are not exactly the same as the ones that gave birth to ancient rocks. Generally, the recent sedimentary deposits are more diverse than those of the initial ocean. Sea water salinity varies from one ocean basin to another, depending on the local evaporation intensity, the contribution of freshwater rivers, or on the underwater volcanic activity.

It is assumed that the ocean was formed with a particular type of atmosphere released at a time when the planet was covered by volcanoes. After a while, the temperature began to fall and when it came under boiling point, it started to rain ... and did not stop. Over a long period of time, it reached the stage where the surface layer of the oceans evaporated because of solar radiation and left behind was a higher concentration of salt. The vapor formed clouds that were taken towards the mainland, where they condensed and fell as rain. Rain washed minerals from the land, transporting them to rivers.

The water that comes from underground contains many minerals. Rivers reach the ocean with all these minerals which are then deposited and over time, they become salty. Even though water still evaporates from the oceans, the minerals remain.

Other salt sources are given by the quite numerous caverns that are on the bottom of the ocean. Over 72 chemical elements were found in the ocean water (in a relatively homogeneous consistency), most of them in very small quantities, but they are probably all the elements that naturally occur on Earth.