Evaporation increases the ocean's salinity by removing fresh water from the ocean's surface, leaving the salts. Precipitation lowers the ocean's salinity by adding fresh water. In addition to carrying salt to the ocean, rivers also carry fresh water, which lowers the salinity of seawater near river mouths. The oldest method of obtaining salt is by evaporating seawater. As seawater is moved through a series of evaporating ponds, the various minerals evaporate before the salt does, leaving a product that is 95 to 98 percent sodium chloride, the encyclopedia says.
Salt also is mined from rock salt, the huge deposits beneath the ground.
This method requires that about half of the salt be left behind as pillars. In solution mining, a well is drilled into the ground and two pipes are lowered into the hole. The pipes consist of a small central pipe inside a larger pipe. Fresh water is pumped down the central pipe to the salt deposit. The water dissolves some salt to form brine, which is pumped to the surface through the outer pipe.