There is a huge deposit that runs from western NYS over into Michigan. There are several mines here in NY, the oldest (Watkins Glen NY) has been in place over a century. That plant uses steam to extract, and the re-crystalizes the stuff. Most of the other mines use underground extraction, they dig it out, then convey it to the surface.
Syracuse a.k.a. Salt City. Quite valuable real estate in the early days for the salt springs. So salt on the roads is a sort of recycling. And they use plenty. First place I heard the term "winter rat." Keep your nice car garaged and drive a rust bucket in winter. Or next winter your nice car will have become a winter rat. One spring day I had to drive to Camillus. Roads clear all the way but could see white when I got to Camillus. WTF? Yup salt. Getting rid of the leftover from winter I guessed.
Trivia: Salt shipped to NYC on the canal system would get chunky from the humidity. The most menial of menial jobs was knocking it apart for the shakers. Hence the dismissive term, "Go pound salt." NOT pound sand - pet peeve.
