During one of the many discussions at CodeMash, I brought up the fact that deep beneath the city of Detroit (some 1200 feet below the surface) exists a very large salt mine that spans some 1500 acres and contains over 100 miles of roadways connecting a series of large excavated rooms. This salt deposit, formed hundreds of millions of years ago when a great salty sea evaporated, actually extends under most of Michigan and lower Ontario, as well as parts of Ohio and New York. A large operation in Cleveland, for instance, mines salt from equal depths below Lake Erie.
My discovery sources: http://www.detroitsalt.com/home.htm , http://www.saltinstitute.org/mich-1.html