But first things first. How did the salt get under the city of Detroit in the first place? For an answer to that question, we have to go back about 400 million years when the first humans weren’t even a speck on the horizon. An area today known as the Michigan Basin was then separated from the ocean and kept sinking lower and lower into the Earth. Salty ocean water kept pouring into it until gradually, the ocean receded, leaving the water to evaporate and huge salt deposits behind.