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, D.C. — the ultimate “undisclosed location.”

Except that with the end of the cold war, the secrets of this government bunker began leaking out.

In the mid-1990s, the chancellor, Helmut Kohl, came under fire after his ministers requested more than $100 million to upgrade the complex with new furniture, fire-protection equipment and communications technology. The government had already decided to move the capital to Berlin from Bonn, and opposition leaders in Parliament cried foul.

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.

Image A tunnel. Credit... Marcus Gloger for The New York Times

Its first move was to solicit proposals from the private sector for other potential uses. The ideas — an amusement park, a restaurant, a disco, or a cultivation center 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.

Mr. Mausbach began that work in 2001 but halted it for a few weeks after the Sept. 11 terrorist attacks in the United States, 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.

Public curiosity about the bunker lingered, however, and Mr. Mausbach said he wanted to find a way to preserve at least part of it for posterity. Restoring the entire complex would have been impossibly expensive, so he hit upon the idea of restoring a small portion of it.