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IT’S EARLY APRIL, and Berlin’s Humboldthain Park is in full bloom. Wide paths run beneath leafy trees, kids play in grassy meadows, and couples sit on benches watching the world go by. At the park’s northern end, between a public pool and a rose garden, the start of an unnaturally steep hill forces me to hop off my bike and walk. The trees get closer, and the paths narrow into switchbacks as they climb. At the top, five stories or more over the train tracks that border the park, there’s a concrete wall—and a heavy steel door.

Waiting for me is Holger Happel, a guide who works for Berliner Unterwelten, a volunteer organization dedicated to exploring and preserving Berlin’s near forgotten subterranean spaces. The hill underneath our feet, he explains, is entirely artificial. We’re standing atop the ruins of the Humboldthain flak tower. Once a castle-like concrete and steel bunker and gun platform 10 stories high, it was completed in 1942 as part of a network of antiaircraft towers that spewed flak at incoming Allied bombers. (The term “flak” is short for Flugabwehrkanone—antiaircraft cannons.) Humboldthain is one of a few fortifications and bunkers remaining in Berlin. Allied engineers removed the most prominent after the war, but local authorities repurposed a few, while others were forgotten or too solidly built to destroy.

Humboldthain is one of the latter: After the war, French engineers blew up half of it and buried the rest under hundreds of tons of dirt and rubble. Technically, Happel’s tour will take me and a gaggle of German tourists underground, but we’ll be high above the park the whole time.

At the bunker’s entrance, Happel hands out hard hats and takes a look at our shoes. “High heels, sandals—we’ve seen it all, believe me,” he says, satisfied that every one’s feet are protected. With that, the door slams shut and we begin our descent.

The tour begins at the uppermost floor, where gun crews once manned the 88mm flak cannons on top of the tower. It’s chilly and a little bit damp. The bunker’s interior is a steady 50 degrees Fahrenheit year round, an ideal habitat for bats in the winter—which is why Humboldthain is only open for tours from April to October.

Work on Berlin’s trio of flak towers began in the fall of 1940, a few months after the first British air raid on the cap ital. “It took less than a year for the war to turn back on Germany,” Happel says. The attack, at the extreme range of British bombers at the time, put Berlin on notice: The defensive belt of antiaircraft guns in the forests around the city wasn’t going to be enough to ward off assaults.

Nazi officials responded with an urgent building project for shelters, bunkers, and flak towers. German leaders made the Sofortprogramm a national priority, sparing no expense. Some of Germany’s top construction firms used prisoners of war and forced laborers to complete the project. Because of Berlin’s sandy soil, most of the construction was above ground.

The flak towers were the program’s most visible component. In 1940, construction began on towers in central Berlin’s Tiergarten region and in the eastern Berlin neighborhood of Friedrichshain. In October 1941, work began on the Humboldthain tower. When it was finished seven months later, the three towers formed what looked like a formidable defense system. The trio coordinated their fire, with shells timed to explode at a preset altitude and create clouds of shrapnel that approaching bombers had to fly through. “For propaganda, it was a big deal,” Happel says. “It let the Nazis show people ‘Look, we’re trying to protect you and do something.’”

But Humboldthain and its brothers were more PR than protection. Over the course of the war, the tower downed 32 bombers—not a terribly impressive number, considering there were more than 300 raids on Berlin, some with more than 750 heavy bombers. Late in the war, high-altitude bombing put planes out of the flak towers’ reach entirely.

The Humboldthain tower never sustained a direct hit. As a result, the top floor is in great shape compared to lower levels, which succumbed to French engineers’ dynamite. In one corner of the building, a spiral concrete staircase hangs by threads of rebar, leaving a gaping six story shaft down the middle. Happel’s flashlight isn’t powerful enough to penetrate the gloom at the bottom. On the third floor, the roof is concave, sagging under the weight of tons of dirt outside. There are gaping cracks in the inner walls and head-high piles of rubble.

The lower levels were designed as a civilian shelter, capable of holding 15,000 people. Most Berliners took refuge from bombs in makeshift basement bomb shelters, subway tunnels, or reinforced trenches. But when the Humboldthain tower finally surrendered on May 2, 1945, between 40,000 and 50,000 terrified Berliners were huddled inside—mostly civilians fleeing from Soviet troops who had overrun the surrounding neighbor hood. “By the end of the war, people were spending days or even weeks in this concrete cave,” Happel says.

On the wall of one cavernous chamber, a laminated 1941 photo shows the tower’s roughly 170-person crew, lined up in rows outside the front entrance. “If you took that photo a few years later, it would look like a high school graduation picture,” Happel says. By 1944, the gun crews were actually high school students, most of them just 17 or 18. They were given armbands emblazoned “LH,” for Luftwaffenhelfer. In the last days of the war, Berliners joked the initials stood for Letzte Hoffnung—last hope.

Most of Berlin’s bunkers didn’t fare as well as Humboldthain, but the cityscape is dotted with concrete remainders of the air war against the German capital. Berliner Unterwelten runs tours in a handful of other World War II-era bunkers, keeping them clean and secured in exchange for permission to enter them.

A few hundred yards from the Humboldthain tower is the Gesundbrunnen U-Bahn station, the site of the association’s most popular tour. This one was purely a shelter, designed for civilians and set up in the warren of tunnels underneath the subway tracks. Open for guided tours, it’s a revealing look at wartime conditions; unlike the nearby flak tower, the bunker never sustained major damage.

Inside, several rooms recreate the crowded atmosphere of the war time bunker, with composting toilets and bunks packed into claustrophobic, low-ceilinged chambers. The highlight is the room reserved for emergency responders, painted floor-to-ceiling with glow-in-the-dark paint. When the guide turns off the light, the room is still bright enough to make out everyone there. “Please don’t touch the walls,” the guide says politely. “The paint’s not very good for your health.”

Across town in Schöeneberg, you can see the remains of a bunker beneath a major housing project on Pallasstrasse. Sitting as it does in a densely-populated neighborhood, the bunker was too difficult to destroy safely. The blown-out Pallasstrasse bunker had a cameo in Wim Wenders’s 1987 film Wings of Desire, a surreal love song to Berlin featuring angels, circus performers, and Peter Falk as a former angel dedicated to persuading celestials to become mortals.

And near the main train station in the city center, there’s the so-called “Art Bunker.” This civilian shelter led a colorful postwar life as a Soviet prison and fabric warehouse. In 1957, the East German government used it to store tropical fruit imported from Cuba, earning it the nickname “Bananenbunker.” After the Berlin Wall fell, entrepreneurs turned it into one of Berlin’s most out there fetish and sex clubs; in 2003 a wealthy art connoisseur bought it to dis play his collection.

The atmosphere inside is more minimalist gallery than battle-scarred haven. Some of the concrete walls have been painted white, hallways are hung with photography, and rooms have been enlarged to make room for works by artists like Olaffur Eliasson and Ai Weiwei. The brightly-lit spaces and murmur from video installations are a total contrast to the cavernous gloom of Humboldthain.

As we file through the lowest level of the Humboldthain tower, it’s hard to imagine I’m still three stories above ground, level with birds’ nests in the surrounding trees. Cascades of shattered brick and concrete spill from the walls. Mounds of rubble are piled to my right and left, leaving a narrow passage to walk through. The stairways are canted at odd angles. In some spots, steel steps bolted to the walls replace collapsed flooring.

For a moment, I’m transported to the desolate fall of 1945. Hundreds of tons of concrete and steel couldn’t protect the city once the air war began in earnest. The conflict conceived and set in motion in Berlin returned to devastate the city. By the time the war was over, all of Berlin resembled this dark, silent ruin.


Originally published in the October 2014 issue of World War II. To subscribe, click here.