I must have walked past it a hundred times. I’d tried getting in once or twice on my initial visits to Schönholzer Heide but it was sealed shut, secure, no way in. I was always left with the taste of defeat in my mouth, but nature generally consoled me as I walked among the trees.

This time, however, one of the metal plates at the back had been ripped away by some determined soul. I couldn’t believe it. It was just a matter of hopping in!

“You wanna go in?” I asked the young fella, who’d wanted to go to the forest in the first place.

“Yeah,” he replied without hesitating. He’s up for anything, that fella.

I went first, in through the gap over the rusted wartime-era radiator, and then lifted him in.

“It’s dark!” he said.

Yes, young man, you’ll get to learn that bunkers are indeed usually very dark.

Luckily I’d brought a torch and we set off to explore it. Bricks, rubble and rubbish littered the floor, as rubbish likes to do. Two chambers ran down its length, one either side of a central passageway.

Old rusty pipes hung from the ceilings and converged at what seemed a fireplace in the middle of the passageway, flanked by an odd-looking assortment of wheels, valves and levers.

The chimney went straight up, so I presume fires were lit here and these heated the water that heated the radiators at the bunker’s extremities. There wasn’t much else but for the aforementioned litter littering the place. And as I mentioned before, it was dark.

It was only afterward that I learned its history was even darker. The bunker is all that remains of the second biggest labor camp in Berlin, operated between June 1940 to April 1945.

It was named “Luna-Lager” (Luna Camp) after the former Lunapark amusement park, which moved to Schönholzer Heide from nearby Halensee in 1933. It must have been someone’s sick joke to name a prisoners’ work camp after a fun park.

The Heide (moor, heathland or heath) had been popular from the end of the 19th century among Berliners escaping the developing city for recreation, with pubs, restaurants and a shooting range.

Its story began in 1751 when Prussian queen Elisabeth Christine, Frederick the Great’s wife, had mulberry trees and fruit trees planted here, and avenues created so people could admire them. It became known as the “Queen Plantation” and belonged to her Schönhausen Palace summer residence.

She died in 1797 and by the time another 100 years had passed the Heide was a much-loved recreational destination. (There are some great pictures of it from times long gone here.)

Health resorts for women and children were built around 1900 to treat tuberculosis, anemia, weak nerves, stomach disease and rickets. Separate incidences, I hope. Tennis courts and football pitches were added in the 1920s and a sleigh slope was added in 1930.

Then came the fun park. Lunapark was renamed Traumland (Dreamland). It was an early version of Spreepark, with loads of attractions, a Ferris wheel and even a rollercoaster called the Himalaya-Bahn. The war put an end to the fun, however, and the whole thing was closed down.

That’s when it became a labor camp. Forced laborers – mostly from Poland, France, Belgium, Croatia, Serbia and “Ostarbeiter” from the Soviet Union – were used to make weapons for the Nazis’ war effort at the behest of the Deutschen Waffen- und Munitionsfabriken (DWM, the German Weapons and Munitions Company).

“On Feb. 19, a very cold morning, the German army enclosed Grabowiec. A German soldier came to every house and gave us 15 minutes’ time. They sorted us then in Zamość. The young and healthy came to Germany, to work … the older people, also those who were sick, were brought straight away to Oświęcim, to Auschwitz. I came with my parents to Berlin then, and we were brought to the munitions factory,” Anna Buczak recalled.

DWM used prisoners to their benefit in many locations. The Berlin factory was at Eichborndamm in Borsigwalde, also in the north of the city. The “employees” had to work 12-hour shifts for six days a week there.

“The work began at 6am and we had to get up at 4 already, because we had to go four stations withn the S-Bahn and because there was so many of us – Russians, French, there were masses of people – that we had to go an hour earlier so there was room on the S-Bahn for the Germans later,” remembered Jadwiga Adamiak. “And so we were at the factory at 5 and then we waited for an hour so we could start working at 6.”

Many of the workers died from malnutrition and disease, and those who didn’t very often perished in bombing raids due to a lack of suitable protection.

Joseph Lenzel, a priest from Niederschönhausen, attempted to help them and for this heinous crime he was arrested on Jan 7, 1942 and brought to the concentration camp at Dachau. He lasted until July 3 before his murder was completed.

There were up to 2,500 laborers by the end of 1942, each identified according to background. Polish workers were marked with a “P” while “OST” identified the Soviets. The Nazis liked to keep their racism labelled.

More than 100 prisoners, including 12 infants or small children, died at the camp between 1942 and 1945, before the survivors were freed by the Red Army. To be honest, I’m surprised more weren’t killed.

Thousands of Soviet soldiers died in the battle for Berlin between April 16 and May 2, 1945 – perhaps more than was necessary – and one of three huge memorials to fallen Red Army soldiers was built at Schönholzer Heide.

It’s huge in true Soviet memorial style and it reminds Germans that “they gave their lives for your future.” Be grateful for your heroes. Strangely, it doesn’t mention the orgies of rape and violence that Soviet troops inflicted on the surviving civilian population.

The memorial, which was completed in 1949, is still there of course, as is a small graveyard for 350 people who died locally in Pankow between April 14 and June 22, 1945. Altogether 180 civilians, 146 German soldiers, 17 refugees and seven foreign workers from Luna-Lager are buried here.

The bunker was one of three apparently. The other two were blown up. Most wartime bunkers met an explosive fate. I don’t know why this one was spared. But it’s the only structure in the whole of Schönholzer Heide to have survived since wartime. Everything else was destroyed.

Now the park is home to joggers, cyclists, picnickers and weekend revelers. They appreciate the open space, the grassy knolls, the wooded areas – Schönholzer Heide is a fine park. Forget your past. Be grateful.