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Over hundreds of millions of years, rain formed rivers – which, in turn, dissolved rocks from different geological periods, in which common salt, sodium chloride (NaCl), is found in abundance. As all waterways flow into the ocean, the seas get almost all of the salt dissolved in the process. In addition, chlorine and sodium particles suspended in the atmosphere are also washed away by rain, completing the process. Still, the salinity of a body of water depends mainly on its evaporation rate, which ultimately determines the salt concentration. That’s why lakes and dams can become salty in very hot regions, as in the Brazilian Northeast. For the same reason, the equatorial seas are saltier than the polar ones. The saltiest on the planet are the Dead Sea, in the interior of Asia, and the Mediterranean. The least salty is the Baltic Sea in northern Europe, which, because of its low salt content, freezes over during the winter.