Much smaller than Drakelow, but very similar in design, six of the structures were probably built to be used as factories or ammunition stores. But as they were cut deep into the hard gneiss rock of the Owl Mountains, their construction would have been much more challenging than digging into the relatively soft sandstone around Drakelow. Most of the hard labour was completed by prisoners of war from the surrounding concentration camps of Walbrzych County.

The centrepiece of Project Riese would have been a far older building – the 13th-Century Ksiaz Castle in the city of Walbryzch. The last private owners were the Hochberg family, who lost possession of the castle in 1944 to the Nazi party after falling into debt. To make the castle suitable for military occupation, two levels of tunnels were bored 53m (175ft) under the courtyard and joined to the train line at Lubiechow by a narrow gauge railway – allowing direct access by rail into the castle. It’s thought that the site would have become a headquarters for Project Riese, from which all six of the factories could be managed – and there is some suggestion that the grand castle would have been used as one of Adolf Hitler’s Fuhrer Headquarters, although with the war drawing to a close, none of this was ever realised.

Cold comfort

Top-secret military construction did not end with World War Two. In 1949, as the Cold War bloomed, the UK government began to build 15 fortified war rooms across the country.

But in the case of the much bigger threat of a nuclear attack, these buildings would not have been enough to protect their inhabitants. They were too small, making them unable to support a workforce for the extended period of time they’d need to remain indoors to avoid the fallout of a nuclear explosion. They also were built too close to the major cities which could have been a target for an attack: five were built in London, for example.