But when the 25-ton steel blast doors clang shut on Cheyenne Mountain, they will close on a bygone nuclear era all but eclipsed by new and different threats, from suicide hijackers to terrorists with suitcase dirty bombs.

The staff of the North American Aerospace Defence Command (Norad) will emerge, blinking in the sunlight, to head for a more prosaic home in an ordinary hanger at Peterson Air Force Base.

After months of speculation about the future of the bunkered mini-city immortalised in Cold War films and pulp fiction, military officials have confirmed that the Cheyenne Mountain facility is to close.

“A missile attack from China or Russia is very unlikely,” Admiral Timothy Keating, the commander of Norad, said. “The threat has changed.”

The complex was built in the mid-1960s as America became more fearful about Russia’s ability to launch an intercontinental ballistic missile at its mainland. Nearly 2 hectares (4.5 acres) of connecting chambers and tunnels were dug out of the mountain under 610 metres (2,000ft) of granite.

Fifteen buildings, two or three storeys high, were constructed inside the mountain. Metal walls and tunnels were built to withstand a five-megaton nuclear explosion within miles. The war room is supported by 1,300 shock absorbers to protect it from direct nuclear attack.

The image of a hidden nerve centre with the task of saving America from destruction loomed large in the popular imagination throughout the Cold War era.

In War Games a teenager played by Matthew Broderick hacks into the mountain’s computer, bringing the world to the brink of destruction.

Dr Strangelove, Stanley Kubrick’s apocalyptic satire, in which runaway technology leads to the end of the world, features a war room like the one at Cheyenne Mountain.

In the post-Cold War era its frightening symbolism has waned. Cheyenne Mountain has appeared twice in the irreverent cartoon South Park, once when it is attacked by a giant Rosie O’Donnell, and another time when Cartman’s mutant binder takes control of the Norad supercomputers.

In reality computer malfunctions at Cheyenne Mountain have brought the world close to war twice. In 1979 a communications failure caused messages warning of a nuclear attack to flash up at air force bases around the world. A year later another stream of false warnings, intended as a test, were sent to government bunkers and command positions. Data from other sources identified the warnings as false.

When Cheyenne Mountain was built, the threat and the strategies for dealing with it were based on the notion of a centralised state enemy, the Soviet Union, countered by a similarly unified defence, the US. As computer technologies were also centralised in a mainframe, defence systems worked from one giant computer deep in the mountains.

But just as the Soviet threat has been overtaken by the threat of global terrorism, so has the mainframe been cast off in favour of networks that owe their security to decentralisation. Cheyenne Mountain was no longer required.

Over two years Norad’s 230 staff will move to Peterson, where the Northern Command, set up in 2002 to monitor terrorist threats, is based.

The complex, however, will not be closed entirely. Commanders will keep it in “warm standby”, ready to be reopened at an hour’s notice — in case the world changes again.

BUILT FOR NUCLEAR APOCALYPSE

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