Deep beneath the Colorado Rockies lies a top-secret fortress designed to keep America's military and political leaders alive if the unthinkable happens.

The Cheyenne Mountain Complex, once the stuff of Cold War nightmares, is a sprawling underground city carved out of nearly 700,000 tons of granite and designed to withstand direct nuclear strikes. 

For decades, it has stood ready to coordinate US defenses in the event of Armageddon.

The last time reporters were allowed inside was in 2018, when Donald Trump was still in the White House. 

Now, amid mounting international tensions, the US military has again opened its blast-proof doors - with NewsNation cameras escorted deep into the classified facility.

Behind three-foot-thick steel doors and past multiple checkpoints, reporters were shown the command post that military officials say could withstand a nuclear explosion 'one thousand times stronger than the bomb dropped on Hiroshima' even from just a mile away.

The complex is more than just a bunker - it is a functioning underground city. It has its own power plant, underground lakes for drinking water, and stockpiles of food to sustain crews for 'a very long time'.

A Subway restaurant even operates inside, proudly calling itself the 'most secure Subway' in the world.

Gen. Gregory Guillot, commander of U.S. Northern Command and NORAD, told NewsNation the facility remains as vital today as when it was first activated in 1966.

'It is truly worth whatever they paid for it back in the early '60s, and we are using it today just as they did the decades before,' Guillot told the outlet.

Asked about modern threats - from Russian jets probing US airspace to Chinese surveillance balloons and cyberattacks - Guillot told NewsNation he was unshaken.

'I don't worry. We got the greatest military members working it,' he said, before adding ominously: 'And we are ready.'

The installation spans 5.1 acres and contains 15 buildings suspended on massive springs to absorb the shock of a nuclear blast.

Known as the 'battle deck,' it would serve as the command hub for US and Canadian forces if disaster struck.

Completed in 1966 at a cost of $142million, the Cheyenne Mountain fortress would cost well over $1billion if built today. At the height of the Cold War, it was billed as 'the most secure place on Earth' - a warren of tunnels and blast chambers designed to protect against Soviet missiles.