Suppose the same thing happened w/ Lake Erie ? There are salt mines under the lake...
Largest salt mine I believe is under Detroit....
I believe that the salt mines under the Great Lakes are the remnants from the ancient ocean that at various times covered parts of what is now the U.S. The Great Lakes were carved out for the most part after the glaciers of the last great ice age carved out parts of the area and then receded. The flow of runoff over a long period left some of what are now the lakes.