Vienna's secrets can be big. One of the largest measures half a million cubic metres, weighs several hundred thousand tonnes, and thrusts more than 55 metres into the sky. Not surprisingly, it's a pretty open secret. Everyone in the city knows about the Flaktürme - flak towers - the huge gun emplacements built during the Second World War to fend off Allied bombers.
All six of these reinforced concrete colossi survive, brooding grey thugs of buildings that sprouted at the heart of a city of baroque cherubs, delicate colonnades and sensuous art nouveau maidens. Being thugs, they have refused to budge, and still dominate their surroundings without a hint of remorse.
Yet the flak towers do not exist as part of "official" Vienna, the tourist temple to Mozart, Strauss and chocolate cake. Other than a brief mention in guidebooks and the occasional academic study, they are invisible. Yet Vienna is, of course, also the city of Sigmund Freud, and these relics of a dark past are poised to burst out of the city's subconscious.
Furious at Vienna's state of denial over the towers and the Nazi era that gave rise to them, a band of students and architectural historians is determined that the full truth about the monsters - one of the largest groups of concrete structures in Europe - should be told.
Built between December 1942 and January 1945 as Allied air raids increased, the towers were ordered by personal decree of Hitler. They were designed by the German architect Fried rich Tamms, who designed much of the Reich's autobahn system and its bridges. Hitler had planned for the towers to be clad in marble after the war and, of course, inevitable victory for the Reich; they would be engraved with the names of the millions who had died for the Führer. Each pair of Vienna's towers comprised a combined searchlight/radar tower and a gun tower. The walls and roofs were up to three metres thick, and inside each tower was a warren of corridors and rooms. Generators and water tanks made the Führer's fortresses self-sufficient.
The flak-tower system was ingenious, but useless. Hitler's strategic masterminds were dismayed to find that American and British pilots overcame the Wehrmacht's ultimate deterrent with ease: they simply flew higher, beyond the reach of the guns, or skirted around them. Similar towers were built in Berlin and Hamburg, with equally dismal results. But they did have some use. Sections of each tower were used as air-raid shelters for civilians, and thousands sought refuge in them as the Red Army closed in on Vienna.
Since 1945, most of the towers have remained locked, with the city and national government holding the keys. The occupying Red Army tried to blow some up, but made barely a scratch. In theory, the buildings are protected by law as historic structures, but they are otherwise ignored by the state. In the 1980s, one was converted into an aquarium and another used for storage, but the others remained frozen in time.
In 2006, a group of students persuaded the city government to open one, the Arenbergpark radar tower, briefly, for an art show. The students overstayed their welcome. Fuelled by a latter-day brand of 1960s-style agitprop activism, they soon began to demand that all the towers be used for art and cultural events and for fostering "historical awareness" of the Nazi era. "Some of us started to have the idea of going further than just an art exhibition," says Marcus Hafner, a philosophy student and leading "open flak towers" campaigner.
The students ran the Arenbergpark tower for several months as a free zone open to all visitors. Inside, they had been astonished to find an extraordinary range of artefacts, undisturbed since the last grim weeks of the battle for Vienna in spring 1945. Clothes, children's toys, news papers, discarded uniforms, medical equipment and helmets lay strewn about, in perfect con dition. Underfoot were scattered what the students took to be toy aircraft, but were in fact models used for plotting the movement of Allied aircraft. On the walls, graffiti in French, Russian and Italian had been left by those who built the towers.
The memory of these workers is at the heart of the controversy over what to do with the towers. They were forced labour, an army of several thousand slaves culled from defeated or embattled nations. The Austrian government has paid compensation to some of the workers, but many received nothing. Intriguingly, among the items reportedly found inside the Arenbergpark tower were detailed documents linked to the day-to-day management of the forced labour army, including their food ration.
But then, last year, the police moved in. Citing safety fears, the city government forcibly cleared the Arenbergpark tower last summer, fencing off its entrances and changing the locks. "It is simply that some people don't want to be reminded about the war," says Hafner. "I would like to see the flak towers become lighthouses for a better future." The campaign to "free" the towers continues, but from the outside.
"In Vienna's historic centre, these towers can be seen as surrealistic concrete architecture," says the architectural historian and flak-tower freedom campaigner Ute Bauer, "but I'm not happy about detaching the war from the architecture. At least one of them has to be turned into a memorial. They have to remind us of the inhuman 'efficiency' the Nazis established."
Bauer, whose book The Vienna Flak Towers In Austrian Remembrance is a groundbreaking cultural study of the buildings, is now working on an oral history research project and methodically picking through some of the documents found inside the Arenbergpark tower. Encouragingly, she is supported by the city government and the Austrian Fund for Indemnification - yet, bizarrely, she herself is barred from actually entering the towers. Big secrets unravel very slowly in Vienna.