The Lost Villages

Ten communities were submerged due to the construction of Saint Lawrence Seaway.

The Lost Villages are ten communities in the Canadian province of Ontario, in the former townships of Cornwall and Osnabruck (now South Stormont) near Cornwall, which were permanently submerged due to the creation of the Saint Lawrence Seaway in 1958.

The flooding
The flooding was expected as the result of the Moses-Saunders Power Dam construction, which began in August 1954. In the weeks and months leading up to the inundation, families and businesses in the affected communities were moved to the new planned communities of Long Sault and Ingleside. At 8 a.m. on 1 July 1958, a large cofferdam was demolished, allowing the flooding to begin. In all, approximately 6,500 people were displaced by the project, 530 buildings moved, and countless homes, schools, and businesses demolished.

Flooded towns and their remainings
Some communities were relocated rather than abandoned altogether. Many materials including several historic buildings from the communities are now preserved in a museum in Ault Park near Long Sault. Some high points of land in the flooded area remained above water as islands and can be visited.

In some locations, a few remnants of sidewalks and building foundations can still be seen under the water, or even on the shoreline when water levels are sufficiently low. Elsewhere, divers can follow the old roads and sidewalks of the towns underwater.

Source 2: http://weburbanist.com/2014/03/10/drowned-towns-10-underwater-ghost-cities-buildings/2/
Images sources: http://www.aaroads.com/forum/index.php?topic=6132.0, https://ccusb.files.wordpress.com/2013/06/dmblog1.gif and http://www.stlawrenceparks.com/camping/campgrounds/long-sault-parkway/