Some 1,200 feet beneath the streets of Detroit, under the north end of Allen Park, Dearborn’s Rouge complex and most of Melvindale, runs 100 miles of subterranean roads over an area of more than 1,500 acres. It is the Detroit Salt Mine and as a Detroit industry it is older then automobiles. As a geological entity, this salt deposit is older even than the dinosaurs.
Created some 400 million years ago during the Devonian Period — a time when the first fish were beginning to grow legs and make their way onto land, and the first seed-bearing plants came into existence — it was the result of ancient oceans pouring into a huge basin, evaporating, and leaving huge amounts of salt behind in the process.
detriot salt mine.
via atlas obscura