naturally occurring mineral salts in rocks....over millions of years the salts have dissolved out into water...the salinity of the Earth's oceans is increasing over time as more salts become dissolved into solution. Of course, a little Ice cappage melting will play havoc with this balance and result in a drop in ocean salinity which in turn will destroy the natural entropy of the ocean systems and lead to Armageddon resulting in Bruce Willis having to drive to the center of the earth in a modified morris minor to collect all the salt and then fly it out to jupiter on a giant banana space ship that....woops, I lost it again.
the first bit's true though.