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The Detroit–Windsor Tunnel was built by the firm Parsons, Klapp, Brinckerhoff and Douglas (the same firm that built the Holland Tunnel).^[6] The executive engineer was Burnside A. Value, the engineer of design was Norwegian-American engineer Søren Anton Thoresen, while fellow Norwegian-American Ole Singstad consulted, and designed the ventilation.^[7][8]

The construction method is immersed tube (sections of steel tube floated into place and sunk into a trench dug in the river bottom), as in the earlier Posey Tube. The tunnel sections have three main levels. The bottom level brings in fresh air under pressure, which is forced into the mid level, where the traffic lanes are located, and the third level is where the engine exhaust is forced into and vented at each end of the tunnel. Total cost was approximately $25 million US dollars. 

The river section of the tunnel was connected to bored tunnels on both banks. The tubes were then covered over in the trench by 4 to 20 feet (1.2 to 6.1 m) of mud. Because the tunnel essentially sits on the river bottom, there is a wide no-anchor zone enforced on river traffic.

Read more > http://en.wikipedia.org/wiki/Detroit%E2%80%93Windsor_Tunnel