The Diefenbunker, formerly known by its military designation, Canadian Forces Station Carp (CFS Carp), is a large underground four-storey reinforced concrete bunker and nuclear fallout shelter located in the rural area of Carp, Ontario approximately 30 km (19 mi) west of downtown Ottawa. Between 1957 and 1961, during the Cold War the Government of Canada led by then Prime Minister John Diefenbaker authorized the Diefenbunker to be designed and built as the Central Emergency Government Headquarters (CEGHQ Carp) in an attempt to ensure the continuity of government subsequent to a nuclear weapons attack by the Soviet Union. In 1994, CFS Carp was decommissioned and closed.

In 1994, the Diefenbunker was designated a National Historic Site of Canada because it is considered the most important surviving Cold War site in Canada. The bunker is important as an engineering achievement and to the critical path method of planning used in its construction. In addition, the Diefenbunker is symbolic of the Cold War, a strategy of nuclear deterrence and the Canadian people's determination to survive as a nation following a nuclear war. The Historic Sites and Monuments Board of Canada (HSMBC) plaque located at the Diefenbunker states:

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.

In 1998, the facility was re-opened as a museum called the "Diefenbunker: Canada's Cold War Museum" allowing the general public year-round access to tour the facility.

In 1958, at the height of the Cold War and the infancy of the intercontinental ballistic missile (ICBM) threat, Prime Minister John Diefenbaker authorized the creation of close to 50 Emergency Government Headquarters (nicknamed "Diefenbunkers" by opposition parties) across Canada. These shelters were part of what came to be known as the Continuity of Government plan, which was meant to protect various members of government in the event of a nuclear attack.

The original site, some 9.7 km (6.0 mi) east of Almonte (45°15′06.66″N 076°19′31.05″W / 45.2518500°N 76.3252917°W / 45.2518500; -76.3252917 (Almonte, Ontario)), was abandoned when ground water proved impossible to remove. An abandoned gravel pit outside Carp was selected instead, where construction soon began in 1959 and was completed by 1962.

The Carp shelter would be the largest of such facilities (over 9,300 m² (100,000 sq ft)) and the only one in the immediate Ottawa area. The underground 4-storey bunker required 32,000 tonnes of concrete and 5,000 tonnes of steel. The structure was capable of withstanding a nuclear blast of up to 5 megatons from 1.8 km (1.1 mi) away. It had massive blast doors at the surface, as well as extensive air filters to prevent radiation infiltration. Although supposedly effective against surface nuclear detonations, the facility was later found to be vulnerable to conventional Bunker buster bombs developed after its construction, as these bombs had time delay fuses that would detonate after they had penetrated deeply enough underground.

Underground storage was built for food, fuel, fresh water, and other supplies. The bunker was built to accommodate 565 people for up to one month without receiving additional supplies from the outside. It included an emergency broadcast studio for the Canadian Broadcasting Corporation and a vault on the lowest level to hold the gold reserves of the Bank of Canada.