BAD NEUENAHR-AHRWEILER, Germany: At the end of a serpentine road here, flanked by pinot noir vineyards, an unmarked door is cut into a hillside. Behind it lies one of the most secret places in the former West Germany: a vast subterranean bunker to shelter the government in the event of a nuclear war.

That door finally swung open this week — nine years after this cold war relic had been consigned to history — as the German government broke ground on a project to turn the bunker into a museum.

“People were very curious to see this secret place,” said Florian Mausbach, the president of the Federal Authority for Construction and Urban Planning, which is overseeing the project. “I was struck by the idyllic scenery outside and the nightmare-like atmosphere inside.”