The weathered concrete doorframe had turned green with moss and damp, and it was so ordinary looking that the expansive tunnel network it concealed was all the more astounding. Under towering woodland and a brilliant blue sky, I trampled along a short track to the bunker's entrance, a spot that would once have been guarded by policemen armed with pistols and hand grenades. Luckily, before long we chanced upon the road leading towards a cluster of dark metal and glass buildings partly obscured in the hillside that's home to the smart new visitor centre. The local driver had never heard of the mysterious bunker, and with no phone signal, we ended up driving around in circles trying to find it. Stepping out of the train station in the small town of Skørping, I jumped straight into a taxi for the short ride to Rold Forest.

It's an eye-opening journey into the heart of a Cold War-era time capsule. Only 50,000 visitors are permitted annually, and access has been limited to small groups of 10 on 90-minute guided tours that explore 2km of the labyrinthine bunker system. After years of preparations, it opened to the public for the first time in February 2023 as a museum. The plan was to run the country from inside this shelter, 60m below ground, and its very existence was kept hushed for decades until it was finally revealed in 2012. Secretly built in the 1960s at the height of Cold War tensions, this is where the Danish government and even the queen would have been evacuated if nuclear war had broken out.

Hidden in northern Jutland's Rold Forest, some 400km north-west of Copenhagen, is the sprawling bunker complex of Koldrigsmuseet REGAN Vest (The Cold War Museum REGAN West).

Nuclear fallout shelter floor plans