A lot of people in Southeast Michigan are not aware of the massive mines underneath their own feet. Under the Fisher Freeway near the River Rouge lies one of the best salt deposits in the world. The salt mine, in operation since the late 1800's, has over 50 miles of tunnels and could supply the earth with salt for millions of years. How deep is it? Try 1,200 feet. The salt deposit is about 40 feet thick and bone dry. Unlike the surface, there are not any rats or cockroaches.
I went down with a tour when I was a young geology student. That tour sold me on the idea of being a geologist. After seeing western geology, I always hated the area for being flat and geologically boring...at the surface anyway. I learned a valuable lesson that day; Sometimes really great geology is hidden just below the surface.
Detroit was covered by a hyper-saline sea during the early Devonian Period (still roughly 400 million years ago). The Devonian is known as the "Age of Fish," but you won't see any fossil fish here.
Nice little cross section from the Detroit News.
A picture of the "room and pillar" mine.
Some of the mineral halite from there is as clear as glass. By the way, crystals like the ones below can be had for about 7 bucks.