Statement of Significance (as of designation - April 19, 1993):
The St. Clair River Tunnel is the first full-size subaqueous tunnel built in North America. Prior to its completion in 1891, a railroad had never been able to pass beneath a river. This accomplishment was not possible until engineers learned to combine three elements of tunnel construction: The shield method of excavating; the cast iron tunnel lining; and excavating in a compressed air environment. This tunnel was the world's first true test of the feasibility of constructing railroad (and later vehicular) tunnels through the soft, porous ground commonly found beneath rivers. Tunnels such as these broke the rail transportation bottleneck caused by the difficulty of crossing the wide rivers upon which most of the Nation's major population centers were located.