UPDATE: More on the Detroit Salt Mines from this February 1959 issue of Jeep News.
I saw a reference to jeeps in Detroit’s salt mines on Facebook. After a some research, I found a few more images. The salt is used exclusively as road de-icer.
- You can read more about the salt mines here: http://blogs.detroitnews.com/history/2000/01/22/the-ghostly-salt-city-beneath-detroit/ and here https://www.mlive.com/entertainment/2017/07/detroit_salt_mines_inside_mich.html
- You can see large images of the mine here.