The Underground Railroad in Canada
From the early 19th century until the American Civil War, settlements along the Detroit and Niagara rivers were important terminals of the Underground Railroad. White and black abolitionists formed a heroic network dedicated to helping free and enslaved African Americans find freedom from oppression. By 1861, some 30,000 freedom-seekers resided in what is now Ontario, after secretly travelling north from slave states like Kentucky and Virginia. Some returned south after the outbreak of the Civil War, but many remained, helping to forge the modern Canadian identity.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.