The Diefenbunker is a nuclear bunker built outside Ottawa, Ontario, Canada. It was built between 1959 and 1961 (at a cost of twenty million dollars, CND) to house Canadian government leaders, civil servants, and military personnel (but not their families) in case of a nuclear attack on Ottawa. It was designed to resist a 5 megaton nuclear weapon detonating 1.8 kilometers away. It could house over 500 people, with supplies for 30 days. The name 'Diefenbunker' came from the prime minister at the time, John Diefenbaker. It is a four story underground building, with a total area of 100,000 square feet. The bunker is composed of 32,000 cubic yards of concrete and 5,000 tons of steel, resting on top of massive springs to absorb any shock.

After being built in 1961, the installation was toured by Diefenbaker. Upon finding out that only himself, and not his family, would be permitted inside the bunker in case of a nuclear attack, he claimed he would never use the bunker. The only other prime minister to ever enter the bunker was Pierre Trudeau, who did a short 30 minute tour before leaving, saying the same thing that Diefenbaker did.

In 1994, the bunker was put out of active service. It is now a public cold war museum, and people can visit it for guided tours. It's very nice and cool inside, so it's a good idea for a very very hot day.