The Skyscraper Hiding Beneath the Waves. The Mackinac Bridge underwater is even more impressive than the part you can see. Beneath the Straits of Mackinac is an engineering system rarely seen by the public. Hidden below the surface are underwater foundations that extend deeper than a 20-story building. Here is how engineers made it possible. The Deep Reach Crews excavated through more than 200 feet of water, sediment, and glacial debris to reach bedrock. At the time, Pier 19 became the deepest u