Road salt is mined. It originated from the evaporation of ancient, long extinct seas. This evaporation created large salt deposits underground. Portions of these deposits are blasted using explosives and then the salt is shoveled into trucks for transporting it to a machine that crushes it into usable pieces.
Rock salt is found everywhere. In the United States, major deposits are found in New York State, Kansas, Louisiana and under the city of Detroit. When planning for usage in winter, most cities order more than they will need. In case of a shortage of salt during winters, it usually takes up to two weeks to receive new shipments.