Robert Conrad knew things could get uncomfortable. There were the guards, the explosions, the dark tunnels. He could easily stumble across a detonation in progress, run into a policeman or even land himself in jail.And yet, in the summer of 1987, Conrad donned a construction worker's coverall and a hardhat and hid his camera, a Praktica model with a 35-millimeter wide-angle lens, in a leather shoulder bag of the type carried by many workers in the German Democratic Republic (GDR) at the time. To lend his disguise verisimilitude, Conrad made sure a Thermos jug could be seen poking out of his bag. He wanted to be absolutely sure to look just like any normal construction worker.

Thus disguised, the photographer snuck up to the fence around the construction site on Berlin's Otto Grotewohl Strasse and climbed over the barrier. Once inside, he had to suppress the impulse to start running. "I walked very slowly across the site, as if on eggshells, so no one would notice me," he recalls. Conrad was uneasy. Where was the entrance into this underworld of dark concrete ruins that had been buried for decades under Berlin's streets? Would he be able to climb down into the infamous "Führer's bunker," where Adolf Hitler shot himself in April 1945?...