I’m not Gus, but was in the road salt business before retiring.
There is a 400 million year old ( give or take a few weeks) dried up sea bed under the NE USA and part of Ontario. It’s under Ohio, Mich, Ontario and NY. And it stretches up to New Brunswick. The depth of that salt bed varies but is about 1500-2000 feet below the surface. That salt is mined by multiple methods. For road salt mine shafts are dug and the the salt is mined by drilling holes into the salt seam, filling them with explosives, blowing it up, them crushing and screening to make a certain size. In some cases, the horizontal mining tunnels reach several miles under Lake Erie, or some of the NY finger lakes. This salt is not pure enough for table salt, when mined by drill and blast. This same salt is however mined for human consumption by evaporative methods. In this case 12” or so holes are bored into the salt deposit from the surface. Hot water is pumped into the salt deposit. The water dissolves some creating salt brine. That is pumped to the surface. Then the water is evaporated off and you’re left with very pure salt. There are some more processes to get it to the right gradual size for human consumption but that’s the gist of it.
There are also drill and blast and evaporative mines in central Kansas, but they are only about 600 feet deep. And there are mines in Louisiana along the coast. The salt deposits there are domes, or more of a vertical deposit vs horizontal in the NE USA. And there are evaporative plant there as well.
In Utah, there is a third mining process, called solar evap. Here Salt Lake water is pumped into a shallow pond. The wind and sun evaporate some of the water increasing the salt concentration. Then that brine is bumped through a sequence of ponds until it essentially solidifies. It is then scraped up and is used for water softening and very small amounts in table salt.
That’s the high level over view of where road salt and other salt come from.
