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