While it's technically possible to tunnel under the Straits of Mackinac, it seems unlikely. Here's how a Mackinac Tunnel would compare to other tunnels around the country and world.
In the early days of highway travel, it was impossible to drive across Canada as there was no road across the north shore of Lake Superior. The county's early version of the Trans-Canada Highway included a segment that ran across Michigan's Upper Peninsula.
There's a good chance you've driven on Interstate 296 in Michigan as you've criss-crossed the state via its freeways. There's an equally good chance you had no idea you've driven on Interstate 296.