The Central Emergency Government Headquarters
Irreverently known as the "Diefenbunker," this structure is a powerful symbol of Canada's response to the Cold War. Designed in the 1950s to withstand all but a direct hit by a nuclear weapon, it was intended to shelter key political and military personnel during a nuclear attack. Fortunately, it never served its intended purpose, although the Diefenbaker government made plans to retreat to its protection during the Cuban missile crisis of 1962. The bunker functioned as the hub of a communications network and civil defence system until it closed in 1994.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.