Started in 1960, a year before the wall went up, the bunker cost $2.5 billion to construct. By the time it was completed in 1972, there were nearly 12 miles of tunnels, 936 bedrooms, 897 offices, and five small hospitals. It had five sections, each with its own electricity and water supply.

It was designed to enable 3,000 people to survive for 30 days after an attack. Among those, presumably, would be the chancellor, who could get here quickly from Bonn, the former West German capital, 18 miles away.

Ringed with barbed-wire fences and hidden among the leafy vines, it was—with apologies to the shadowy outposts lurking in the countryside around Washington, DC—the ultimate “undisclosed location”.

Just maintaining the bunker cost $14 million a year and required the work of 180 employees. Unable to defend its expensive dinosaur, the government decided in 1997 to abandon the complex.

Its first move was to solicit proposals from the private sector for other potential uses. The ideas—amusement park, a restaurant or a cultivation centre for mushrooms—seemed fanciful. So the government decided on Plan B: strip out the contents of the tunnels and seal them forever, returning the site to as close to a natural state as possible.

Mausbach began that work in 2001 but halted it for a few weeks after the September 11, 2001, terrorist attacks in the US while the government deliberated whether the bunker might have a use after all. “They concluded that a big bunker was not suited to this new kind of terror,” he said.