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After a swim in the sea , you only need to touch your lips with your tongue . They taste of salt , and a mouthful of sea water is far too salty to drink . So where does all that salt come from ? Surprisingly , the story begins on land . Rain picks up a little carbon dioxide from the air , which makes it slightly acidic . Over thousands of years , this weak acid slowly breaks down rocks and mountains . Tiny pieces of rock dissolve in the water , invisible but still there . Rivers carry these dissolved minerals down to the sea . River water tastes fresh because it contains very little salt , and new rain keeps refilling it . The sea , however , has been collecting that small amount of salt for millions of years . Sodium and chloride make up more than 90 percent of it , and together they form ordinary table salt . Some of the salt arrives from below . On the ocean floor , sea water sinks into cracks and is heated by hot rock deep underground . It comes back out through hot springs on the sea floor , carrying iron , zinc , copper and other minerals . Scientists first saw these springs in 1977 . Then the sun finishes the job . It heats the surface , and water evaporates into the air , but the salt can not evaporate with it . So the salt stays behind , while rain and rivers keep bringing more . Today , an average litre of sea water holds about 35 grams of salt , or 3.5 percent of its weight . Where evaporation is strongest , the water becomes saltier . The Dead Sea has no outlet , so its water can leave only by evaporating . Today it is almost ten times saltier than the ocean . Next time you taste salt on your lips , remember that it once belonged to a mountain .
