This is in marked contrast to the early postwar period, when a veil of silence shrouded the existence of the more than 300 bunkers that survived this dark chapter in German history.

Former East Berlin officials, in particular, turned a blind eye to the existence of buildings, bunkers and tunnels with a wartime connotation. For decades, citizens were kept in ignorance, for example, of the precise location of Hitler's bunker, where he committed suicide in 1945 when the Russians seized the city. A large hump in the infamous "no man's land" near the Berlin Wall at Potsdamer Platz, created when Soviet Army troops blew up the bunker in the late 1940s, was the only visible clue to its existence. The bunker was levelled in the 1980s, and a kindergarten was built on the site.

But it was not until last June that the Berlin Underworlds' Association, a non- profit group founded 10 years ago, put up a sign pinpointing the notorious underground bunker site. In addition to maintaining an underground history museum, the Underworlds' Association arranges special tours of remaining World War II bunkers and shelters in and around the city.

Some 1,000 underground bunkers were built during World War II to protect German citizens. Many lay in ruins at war's end after being targeted by Allied bombers and Soviet artillery fire. But almost a third survived, some virtually unscathed.

Ingeborg Junge-Reyer, the city's senator for urban development, praises the work of the underworlds organization: "Not many people know how Berlin looks underground. It is not accessible for the most part. But they do have an interest in the history of their city and like to take a look at the city's tunnels and vaults."

One prime example of bunker architecture is to be found at the corner of Albrecht and Reinhardtstrasse, not far from the Deutsches Theater. Built in 1942, mainly as a refuge for 2,500 German railway workers during bombing raids, its floors were divided into eight chambers. By late 1944 more than 5,000 Berliners sat huddled within the meter-thick walled bunker, with its narrow window slits, as Allied raids intensified.

Three years ago Christian Boros, a Wuppertal entrepreneur and art collector, bought the bunker building. Now it sports a luxury penthouse suite and garden, a swimming pool, and numerous art exhibition rooms.

Tempelhof Airport, one of the world's biggest terminals, featuring arc-shaped hangars designed in the 1930s to resemble an eagle in flight, also possesses a huge web of underground bunkers, in addition to an aircraft assembly line running four miles beneath the surface. It is due to shut down as an airport in 2008, though the building itself is protected as a historic landmark.

In the early 1970s, the then West Berlin authorities began refitting some of the underground bunkers as nuclear fall-out shelters. One of the best known is at the Pankstrasse U-Bahn subway station, where a steel door leads to a bizarre underground concrete shelter worthy of a John Le Carré novel. Capable of sheltering thousands from nuclear attack, its storage rooms contain bunkbeds, blankets and polyester jogging suits for 3,500 people. This sprawling network of underground shelters is also equipped with wicker cradles, baby carriages and food rations.