The St. Clair Tunnel
This was the first subaqueous tunnel in North America and one of the great engineering feats of the 19th century. Built in 1889-1891 to link the Canadian mainline of the Grand Trunk Railway with Chicago, the tunnel is 1,837 metres long with a 6-metre bore. Joseph Hobson, a Canadian engineer, designed and supervised its construction. His innovative combination of cutting shield excavation, cast iron tunnel lining, and compressed air broke the transportation bottleneck caused here and elsewhere by the impossibility of tunnelling through soft riverbeds.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.