At the close of the Second World War, Berlin was strewn with 17 million cubic metres of rubble. Of this mostly building debris, 1.6 million cubic metres remain today within the city's only accessible war-time anti-aircraft tower - what's known as the Humboldthain flaktower.
It's no surprise then that you must wear a hard hat, good shoes and warm clothing to join a tour called "From the Flak Towers to the Mounts of Debris," a 90-minute visit into this seven-storey subterranean ruin. This is one of half a dozen guided tours run by the Berlin Underworld Association (Berliner Unterwelten e. V.).
The Berliner-Unterwelten office is located outside the Gesundbrunnen U-Bahn and railway station, in the north of what was formerly West Berlin. From the tour office it's a short walk through a park (built entirely over rubble) to one of three anti-aircraft towers, or flaktowers, built in Berlin by Hitler in the early 1940s.
Stepping through a concrete-frame entrance near the dome of the mostly buried structure, we're exhorted by tour guide Henry Gidon, a young historian, to stay away from the edges of metal grate flooring, and stick closely to the narrow paths that wend up and down the semi-negotiable slopes of two upper storeys.
From where we stood, we could peer down into the semi-darkness - five or more storeys of unsafe ruin, like a series of man-shaped caves. Stalactites hang from ceilings. The treacherous interior is now used to train rescue dogs. Colonies of mouse-eared bats have also turned it into a nature reserve for bat aficionados.
Used by the Luftwaffe to defend against Allied air raids, this flaktower and air-raid shelter was capable of holding 50,000 people. The walls are up to 3.5 metres thick; the roof, at 3.8 metres, held anti-aircraft guns with a range of 14 kilometres and a 360-degree range of fire. The structure was, of necessity, almost indestructible. For the most part, Allied aircraft avoided the towers. Yet ironically, Humboldthain gunners shot down only 32 enemy planes over three and a half years, Gidon said.
Archival photos posted on interior walls show war-time medical teams - children were born in the flaktower hospitals - and groups of "luftwaffenhelfer" or "air-force helpers" - boy-soldiers as young as 14 who were co-opted late in the war when few adult males remained available.
During the Battle of Berlin in April, 1945, roughly 30,000 Berliners took refuge in each of the city's flaktowers, among the last sites to surrender to Allied forces.
After the war, Scottish engineers - world-class explosive experts - failed to bring down the Humboldthain. Since 2003, volunteers have spent 8,000 hours removing 1,400 cubic metres of cement and steel, making it somewhat accessible.
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