Now a unique museum that doubles up as the world's biggest escape room, the huge bomb shelter known as the 'Diefenbunker' was built between 1959 and 1961 to safeguard hundreds of Canada's key federal officials in the event of a nuclear attack. The four-storey underground complex was kitted out with everything from an emergency government HQ to living quarters, a cafeteria, a medical centre and even a cavernous bank vault to store gold reserves.

Click or scroll through to explore this little-known Cold War relic and discover its fascinating history...

This unobtrusive metal shed looks like any ordinary outbuilding, if you ignore the radioactive symbol painted on its sheet metal roll-up door. But this contains the entrance to a former top-secret government facility, that's come to be known as the Diefenbunker, which is as big as a department store and extends down over four subterranean levels.

The 100,000-square-foot facility was constructed in less than 18 months and operated from 1961 to 1994 when it earned National Historic Site status. It was repurposed as a museum foundation in 1997.

In 1956, the Liberal government appointed a working group to assess Canada's preparedness for a nuclear attack. Published in January 1957, its final report called for the construction of a classified emergency government bunker near the nation's capital of Ottawa, plus a string of smaller shelters in the provinces. Progressive Conservative Party leader John Diefenbaker was elected prime minister in June 1957.

With the Cold War heating up and the advent of increasingly powerful Soviet hydrogen bombs and inter-continental ballistic missiles (ICBMs), the new PM accepted the report's recommendations and green-lit the federal government bunker and around 50 smaller regional shelters.

Diefenbaker, pictured second from the left at a 1957 NATO meeting, signed off on the construction of a network of government shelters. The initiative was officially codenamed Project EASE (Experimental Army Signals Establishment) and the bunkers were referred to as Emergency Government Headquarters (EGHs). 

In 1958, work began on the flagship shelter in Lanark County outside Ottawa but it had to be abandoned after groundwater flooded the site. A new location was chosen in nearby Carp, situated in a natural valley for added protection around 20 miles from the centre of the capital; close enough for a hurried evacuation.

Construction of the Central Emergency Government Headquarters (CEGHQ) started in 1959 and lasted 18 months. An engineering triumph and the first Canadian project to use the time-saving Critical Path Method (CPM), it involved a staggering 32,000 cubic yards of concrete and 5,000 tons of steel. 

A model atomic bomb and two dummy missiles are on display in the entrance building, symbolic of the threat that justified the Diefenbunker's construction.

No expense was spared on the shelter. According to The New York Times, the bunker cost an estimated CAD$22 million in 1961, which translates to CAD$223 million (£130m/$164m) in today's money. 

The Diefenbunker was built to withstand a five-megaton nuclear blast just 1.1 miles away. Designed to absorb much of the force of the explosion and divert it away from the bunker proper, the shelter's blast tunnel is an impressive 387 feet long.

It appears to be made up of two parts since the doors to the actual bunker are located at a right-angle midway through the tunnel. This would ensure that in the event of a nuclear attack nearby, the shockwave from the blast would pass through the tunnel, minimising the effect on the bunker itself. Plus, the actual shelter was enveloped in gravel, which would allow it to displace a few inches and stay intact.

On top of being shock-resistant, the bunker was designed to be radiation-proof. It was intended to accommodate 535 officials for 30 days, the estimated time it would take for radiation to fall to an acceptable level and enable the Diefenbunker's inhabitants to return to the surface. 

According to Canadian Cold War expert Dave Peters, the complex was contained within a concrete box measuring 154 feet on each side, with approximately 100,000 square feet of space distributed across four levels ranging from the highest floor (400 Level) running down to the lowest level (100 Level), which was used for mechanical systems, storage areas, maintenance facilities and military sleeping quarters.

To get through the nuclear blast-proof steel doors, which are 14 inches thick and weigh a hefty two tonnes, staff and visitors were issued with a colour-coded badge denoting their level of security clearance.

While the bunker was thankfully never used for its intended purpose, it was a hive of activity for several decades following its completion in 1961. Benefiting from a protected communications network, the shelter also operated as Canadian Forces Station (CFS) Carp, with a staff of up to 150 people, and was the site of some of Canada’s most top-secret communications during the Cold War.

The Decontamination Chamber was installed just after the entrance to the bunker proper. If someone had to access the shelter following a nuclear attack, they would be directed to the lead-lined space to take a shower fully clothed.

The new arrival would then have to remove their toxic clothing to dispose of in a chute and take another very thorough shower, before having their radiation levels checked with a Geiger counter. A low reading would ensure entry. If it displayed unsafe radiation levels after several showers, the official would be put in isolation for three days and subsequently retested.

The Diefenbunker has 358 rooms in total. The uppermost fourth level – called the 400 Level – housed several facilities, including the Medical Centre, which was equipped with an operating theatre, infirmary and isolation centre.

Packed with essential medicines, it served as an ordinary medical clinic during the operational years of CFS Carp but was fully outfitted for a nuclear attack.

For example, the three beds in the infirmary were chained to the floor to limit movement following a nuclear explosion. The atmospheric pressure in the clinic was kept lower than the other parts of the bunker to prevent the spread of disease.

The medical centre was also equipped with two confinement cells intended to house patients with high radiation levels or serious contagious illnesses, as well as anyone exhibiting dangerous behaviour to themselves or others. If a nuclear attack struck, officials wouldn't have been permitted to bring their families.

Just along from the Medical Centre is the bunker's Message Control Centre. Encrypted incoming and outgoing messages were sorted in this room. It features a wall slot connecting to the next room, where cryptographers decoded and encoded the messages.

Note the décor: while spartan and functional, much of the furniture in the bunker was brightly coloured to help cheer up the occupants. Other wellbeing-boosting design tricks were used, including painting vertical stripes on the pillars that pepper the complex's corridors in an attempt to make the ceilings seem higher.

Also on 400 Level are the Teleprinter Maintenance Room, Communications Room, Emergency Transmitter Room, a very James Bond-esque Emergency Escape Hatch and the Emergency Radio Room.

The Emergency Radio Room would have been used to communicate with other bunkers in Canada and the US if a nuclear disaster had struck. Other parts of 400 Level now serve as exhibition and educational spaces, including the former Governor General's Suite. 

To ensure the continuity of government, each of Canada's 13 ministers had an outer office for aides and two inner offices on the floor below called Level 300.

While highly classified, the bunker turned out to be the nation's worst-kept secret. From the get-go, rumours of its true nature abounded and in 1961, the year of the shelter's completion, a reporter from The Toronto Telegram blew its cover and spilled the beans to the Canadian public. The reporter George Brimmell also coined the facility's nickname, the Diefenbunker, to poke fun at the project, and the moniker stuck.

The public's reaction to this exclusive shelter for the political elite was overwhelmingly negative. Not long after, Diefenbaker confirmed the bunker's genuine purpose, then attempted to distance himself from it, vowing never to step foot in the installation.

Here's the Prime Minister's Suite; made up of four interconnected rooms there's an additional office for the PM's secretary, a simple bedroom and a basic ensuite bathroom.

A later addition, the Ottawa Semi-Automatic Exchange (OSAX) was installed in the 1980s. Brimming with Burroughs 4800 mainframe computers, it handled more than 100,000 covert messages per month.

The most top-secret room in the complex, the exchange was enveloped in metal to prevent electronic eavesdropping, and accessing it required the highest level of security clearance.

According to the virtual tour, each of Canada's government departments had an office on 300 Level. This one accommodated the Energy, Mines and Resources Department.

Like the other spaces in the bunker, this photo shows a recreation of how the office looked in the 1980s because most of the facility was stripped after its closure in 1994. However, one room that we shall come to shortly was overlooked.

The Industry, Trade and Commerce departmental office was also recreated following the bunker's decommissioning in 1994.

Other restored departmental offices include Transport and Public Works. If nuclear war had happened, each department would have had a staff of up to 30 providing round-the-clock assistance to the respective minister.

Several crucial government agencies had offices on 300 Level including the Canadian Meteorological Office and Central Mortgage and Housing Corporation.

Diefenbaker kept his word and during the shelter's years of operation, only one prime minister visited, Pierre Trudeau, who toured the complex in 1976. Seemingly unimpressed, he slashed the Diefenbunker's budget following the trip.

The two-room Emergency Government Situation Centre (EmGovSitCen) is recreated as it was in 1984.

The centre would have proved invaluable during a nuclear conflict because it was tasked with tracking casualties and assessing damage. Staff would have collated, sorted and analysed data from a variety of sources, including NATO, relaying the information to government decision-makers.

The Federal Warning Centre (FWC) had an equally important role. The centre was connected to the North America Air Defense Command (NORAD) HQ, which was set up in 1957 to provide advance warning of a nuclear attack on the US and Canada.

NORAD would have warned the FWC if a strike was incoming. The centre's personnel would then have activated Canada's Attack Warning Sirens and the Emergency Broadcasting System, informing the nation of the impending attack. Further messages would have been broadcast to radio stations in the country via a dedicated CBC Studio on the same level.

This very 1980s-looking beige office is the recreated Cabinet Secretariat.

Described by the museum as "the Privy Council, the Treasury Board and the Prime Minister's Office all rolled into one", the Secretariat would have coordinated the flow of information to and from the War Cabinet and provided support for its high-level decision-makers.

Located next door, the War Cabinet Room would have been the bunker's nerve centre in the event of a nuclear war.

Up to 12 members of the War Cabinet  –  most likely made up of the PM and top ministers – would have convened in this space to manage the country and issue vital life-or-death directives. Fortunately, it was never used for this, but did come close in 1962 during the touch-and-go Cuban Missile Crisis when the government made plans to retreat to the bunker.

The Women's Quarters are also located on 300 Level, marked by a sign at the entrance that reads “Out of Bounds to All Male Personnel". This restricted area was originally made up of 20 dorms, with space for 110 women. They worked in several departments, from nursing and admin to cryptography.

By the late 1970s, however, no attempt was made to distinguish between genders in planning the allocation of sleeping quarters, according to a 1999 museum training guide featured on Dave's Cold War Canada blog. Now, let's head down to 200 Level.

The largest room in the facility, the cafeteria was open 24/7 throughout the bunker's years of operation. It served four meals a day to the permanent staff of up to 150 and would have been able to sustain 535 personnel for a month in the event of a nuclear war.

Not just a place to eat, the cafeteria was the social hub of the bunker. It was kitted out with a leisure area where officials could play pool, darts or shuffleboard, and partake in other fun activities.

That said, the food was no doubt the main attraction. As alluded to by Maclean's, the cafeteria's menu was voted the second-best in the Canadian military, second only to a post in Quebec famed for its delectable poutine.

200 Level also housed the bunker's laundry room, sleeping quarters for senior military officers and quarters for lower-ranked male personnel. Now, let's make our way to the bottom 100 Level.

The lowest level housed various facilities including a small armoury. As we've mentioned, the Diefenbunker was decommissioned in 1994. The Cold War had ended three years prior and the shelter was therefore deemed obsolete. Its contents were stripped, but this particular male dorm, the so-called Forgotten Bedroom, was overlooked.

Moving on, the cavernous Bank of Canada Vault on 100 Level is the most secure area of the bunker. Accessed through an extra-thick steel security door, it would have stored the nation's gold reserves had the unthinkable come to pass. Luckily, it was never used for this purpose and was converted into a gym in the 1970s.

Following the Diefenbunker's decommissioning, it was designated a National Historic Site of Canada. The site was eventually sold to a grassroots group keen to preserve it for posterity and it reopened in 1998 as the Diefenbunker: Canada's Cold War Museum.

Fresh food, which was delivered every week, was kept in the bunker's 100 Level Cold Storage, which, rather gruesomely, would have also served as the facility's morgue during a nuclear war.

Like many other rooms in the bunker, it has been restored and is a recreation of how the space would have looked during the 1980s.

100 Level is also home to the Main Machinery Room, which housed the generator, boiler, backup computer and other tech required to power the bunker.

Since its founding in 1998, the museum has gone from strength to strength and now attracts tens of thousands of visitors a year. It hosts the world's biggest escape room, as well as other events, including corporate bashes, birthday parties and weddings and has been used as a movie set. And thanks to several recent funding injections, the Diefenbunker's future as Canada's leading Cold War heritage site looks very much assured.

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