The world’s oceans taste of salt because rivers have been delivering dissolved minerals to them for billions of years. Rain falling on land is almost fresh. As it runs downhill it weathers rock, picking up sodium, chloride, magnesium, calcium, and potassium. Those ions reach the sea and stay there. Water evaporates from the surface and returns as rain, but the salts do not fly with the vapor. Over geologic time the oceans have concentrated what the continents slowly lose.

Not every salt arrives at the same rate. Hydrothermal vents along mid-ocean ridges add and remove chemistry as seawater cycles through hot crust. Some ions precipitate as sediments or are locked into clay and limestone. The present mix is a long bargain between input and burial, not a simple pile of table salt. Average seawater is about 3.5 percent dissolved solids, enough that a cubic kilometer of ocean holds tens of millions of tons of salt.

Salinity is not perfectly even. The tropics can be saltier where evaporation is strong and rain is scarce. Near great river mouths and in polar regions where ice melts, the surface is fresher. Life depends on that range staying relatively stable. Cells, blood, and the chemistry of shells all evolved inside a salty bath. If the oceans had remained as fresh as rain, the history of animals would look very different.
