Why America Spends $500 Million on Salt It Already Has

America imports about a third of its salt, spending roughly half a billion dollars a year on it, while 1,200 feet beneath Detroit, crews are still working inside a mine most of the city has never seen. More than 100 miles of tunnels run under the neighborhoods above, and the only thing keeping those streets from sinking into the hole is the salt the miners leave behind. How did a mine this size end up under a major American city, why is America still buying salt from abroad, and why does almost nobody get to go down there?