As a child, my grandmother brushed her teeth every day with radioactive toothpaste. It was called Doramad and the packaging promised a ‘wonderful lather’ that would reveal ‘sparkling, brilliant teeth – radioactive brilliance!’ She lived in the small town of Oranienburg, thirty kilometres north of Berlin, where her father was the chemist in charge of making it for the Auergesellschaft. Since he was prone to bleeding gums and abscesses, he had also volunteered to be one of Doramad’s first test subjects. A colleague had injected his upper gums with Actinium-228 and Radium-228 and for two days his whole mouth had throbbed and swelled but then, suddenly, the inflammation disappeared and the abscess closed. Once Doramad went into commercial production in 1925, he brought tubes of it home to his family. Their third-floor apartment was so close to the factory that, after brushing her teeth at night, my grandmother would lie in bed and listen to the churning of the autoclave.

On the morning my wife and I arrive in Oranienburg, ninety-four years later, the local authorities are draining the river. A 500-kilo bomb has been found buried in the soft banks of the Havel and they are lowering the water level to reach it. It is a big operation; a row of pipes like a church organ slurp at the muddy water. Five thousand people are being evacuated from their homes while a hundred police and firefighters establish the perimeter. This is the 209th bomb from the Second World War to be found in this town since German unification in 1990 and authorities estimate that as many as 400 remain beneath the ground. That’s one for every hundred residents.

Most of the bombs have chemical fuses that were supposed to delay detonation by a few hours or days but ended up working to a schedule of decades. Whether it’s now or fifty years from now, they will all go off eventually. In November 2013, Gunthard ‘Paule’ Dietrich returned from walking his dog to find his home was gone and in its place a twenty-metre crater was pooling with brown water. The authorities had hoped to defuse the bomb buried in his garden but, in the end, saw no choice but to detonate. The man in charge was André ‘the Blast Master’ Müller, the head of the Oranienburg bomb squad. He is something of a local celebrity. With his pot belly, grey moustache and zip-up fleece, he resembles an off-season Poirot. Along with Mr Dietrich, he is one of the stars of The Bomb Hunters, a documentary about life in ‘the most dangerous town in Germany’.

We walk north, away from the cordon, towards the local archive. It is housed in a former stately home – one of the few noteworthy buildings to have survived the Second World War – and there we meet the archivist, Christian Becker. We are hoping that he can help us find out about my family – my grandmother, Dorothea, her brother and her parents – who lived in Oranienburg until 1935 when, as Jews, they fled to Turkey. They left behind much of their money and their belongings and it was only during the 1936 Olympics, when they hoped that the presence of the world’s press would limit Nazi aggression, that they decided to return to Berlin and Oranienburg in order to conduct a heist on their own home.

Christian has a colourful silk scarf wrapped round his neck. For someone who has spent twenty-one years attending to a basement archive which details Oranienburg’s role in one of the darkest periods of human history, he is surprisingly chipper. When he sees us, he expresses surprise that we are ‘so young’. We sense this says more about his usual clientele than it does about our youthfulness.

Leading us down into the low-ceilinged basement, Christian talks about the bomb in the river as we might talk about the weather. He tells us that whenever there’s an evacuation, the kids are disappointed if they don’t get a day off school. He explains that Oranienburg is the only town in Germany that is actively searching for unexploded bombs. And every few weeks, they find one.

The Allies had many good reasons to target Oranienburg. Military aircraft were built here, there was a laboratory to develop new chemical weapons and a factory that made gas masks, including the ‘J’ filters that the SS used when operating the gas chambers. In 1933, one of Germany’s first concentration camps – largely for political prisoners – was opened in an old brewery in the centre of town. In 1936, a bigger second camp, Sachsenhausen, was built on the northern edge of Oranienburg. Two hundred thousand people were interred there; 100,000 died. From 1940, a secret facility produced one tonne of uranium oxide each month as part of the Nazis’ attempts to develop nuclear weapons. In 2017, a local metal detectorist happened across a rock with unusual properties and took it home to show his children. Two days later, his street was evacuated while government workers took away the radioactive lump in a lead-lined suitcase. In other words, Oranienburg is the town where Berlin hid its secrets.

In the context of all this, my great-grandfather’s involvement in Doramad toothpaste seems almost quaint until Christian explains where the radioactive ingredients ended up. The thorium used to make the toothpaste was extracted from radioactive monazite sand which had been imported from Brazil. Huge piles of it lay around the grounds of the factory. When the people of Oranienburg started to rebuild the town after the war, they filled the bomb craters with whatever material was readily available. In this way, the foundations of Oranienburg are now both radioactive and explosive. It is not unusual for André Müller and his team to wear hazmat suits while they dig.

Disappearing into the rows of shelves, Christian finds a directory of postal addresses from 1931–2 and is pleased to discover my great-grandfather’s name, Siegfried Merzbacher. He and his wife Lilli lived at Lindenstraße 15. Before I can ask whether that building still exists, Christian shows us a large poster pinned to the wall: a composite of aerial photos of Oranienburg taken by the American and British bombers after the huge raid on 15 March 1945, when over 4,000 bombs were dropped in forty-five minutes. Oranienburg is a moonscape. These aerial photos are what André Müller and his team use as a map in their attempts to locate the remaining live explosives. Craters show where bombs went off and smaller keyhole dots indicate where they didn’t – and where they might still be buried. Christian points his finger at a rash of craters and says, ‘Here’s Lindenstraße.’

I shelve my hopes of ever standing in my grandmother’s childhood bedroom. On the upside, however, Christian does find a box file that contains the architectural plan of their apartment. They lived in a building that was built and owned by Auer, my great-grandfather’s employers, a company that had made its name selling a mildly radioactive gas lamp – the Auerlicht – before diversifying into toothpaste, gas masks and radioluminescent paint. Christian unfolds the crumbly pages, revealing a grand three-storey corner building with a steep red-tiled roof, high windows and a panelled front door flanked by columns. A vertical section shows the inside of the building. I peer into the attic and wonder if that’s where they hid the jewellery box that contained the bloodstone ring that I’m now wearing. It’s a story I first heard from my mother when she gave it to me, how this was one of the items my family smuggled out of Nazi Germany in 1936.

She never gave me any more detail than that and yet I’ve been retelling the story ever since, making it more dramatic each time I do, so that now Siegfried’s palms are slippery on the wheel as they head south through Berlin in the middle of the Olympics, past sports fans with flags around their shoulders, past huge swastikas lining the Lustgarten, storm troopers on every corner – his driving becoming suddenly self-conscious, taking each turn with elaborate care – and it is only many hours later – as Jesse Owens, an African American sprinter, breaks the world 100-metre record while the Führer looks on, seething – that Siegfried Merzbacher, portly Jewish chemist with his kids in the back and an unusually heavy briefcase, is waved across the border into Czechoslovakia.

Christian returns with another box file. He has discovered that there was also an air-raid shelter in the basement of the building, installed in 1932. This is surprising, he explains, because typically air-raid shelters were not built in Germany until 1938 or later. We ask why they would have singled out the residents of this building for special protection but, for the first time today, Christian does not have an answer.

When we get back outside, a siren sounds. Minutes later, the protective cordon is lowered and residents return to their homes. Photos on social media show André Müller with his hand resting proudly on another disarmed American bomb. It is orange with rust and resembles a whole roast pig, an oilcloth stuffed in its mouth. We watch the mayor hand Müller supermarket flowers and thank him because there is now ‘one less monster in Oranienburg’. Only 400 monsters to go.

Walking through the quiet, wide streets, we try not to think about the homes built on radioactive sand, the lumps of uranium found lying in the long grass or the unexploded bombs beneath our feet, nose up. It probably doesn’t help that my wife is pregnant. Next week we’ll be going for our ‘anomaly scan’. We envisage the doctor asking, ‘So remind me why you spent a long weekend in Germany’s most radioactive town?’

At the edge of a football pitch, a small grey sign explains that Auer built extra factories here to meet the increased demand for gas masks as the country prepared for war. Then, from 1943 to 1945, the factory buildings were taken over to extend the Sachsenhausen concentration camp. Where we are now standing, as many as 2,000 female prisoners lived in barracks, forced to make masks that would be used to protect the SS while they operated the gas chambers. On 15 March 1945 – the day thousands of bombs fell on Oranienburg – hundreds of these prisoners were killed. They had no air-raid shelters. We look up from reading the sign to see young footballers in tracksuits doing shuttle runs, weaving between cones, their breath huffing in the cold. Beyond them, steam rises from a sports complex: swimming pool, sauna and wellness centre.

A few minutes’ walk away, we find an astroturfed playing field that was once the cobbled playground of my grandmother’s old primary school. The school has changed names since she was here but it remains on the same site. In 1933, when she was nine years old, a new law limited the number of Jewish children in each school to 1.5 per cent of the total intake. Since she was already the only Jewish child in her class, life continued as normal for her. Nevertheless, the school’s director, Professor Katz, went out of his way to promise her that she would be treated no differently to any other child. At that time, Professor Katz was still flying the black, red and gold flag of the Weimar Republic above the school, telling the parent–teacher association that he would not withdraw it at the request of ‘any association of goatherds’. A few weeks later, a group of those goatherds arrived at the school in SS uniforms, burned the flag and raised the swastika. From then on, each morning, Dorothea stood in silence while her fellow students saluted the Führer and sang anti-Semitic songs. She told me that on occasion she found herself singing along with some of the less offensive ones, ‘but only because the tunes were good’.

Their apartment was just around the corner from the school, at the junction of Lindenstraße and Lehnitzstraße. On one side of the road – where the toothpaste factory was – is now a Park & Ride car park. On the other side, where the apartment stood, there are bland, prefabricated blocks built in the 1960s when Oranienburg was part of East Germany. Between two of these buildings, we notice a fenced-off area that looks like an ordinary construction site. Then we see the rows of regularly spaced bore holes that mean the bomb squad have been here, digging.

While peering through, we do some research on our phones. My wife learns that Oranienburg remains a popular weekend destination for the amateur radiation spectrometrist community. They like it for the soil samples. I discover a survey by the Federal Office for Radiation Protection from 1997. As well as highlighting the places where bomb craters were filled with monazite sand, it picks out three areas with greatly increased radioactivity in Oranienburg. The first is on the canal at the edge of town. The second is where we’ve just been, the football pitches and sports centre. The third is where we are right now.

Without a word, we start walking back to the train station.

That evening, in a vaulted room within the huge onion-shaped dome of Berlin’s central synagogue, we stand to have our picture taken beside a life-size image of my late grandmother. She’s part of an exhibition about former refugees who, from 1969 onwards, returned to Berlin at the invitation of the city, to see how much it had changed and to tell their story to younger generations. At the height of the programme in 1987, Berlin’s Senate chartered planes and flew nearly 2,000 former refugees from as far away as New York, Buenos Aires and Tokyo. The survivors were greeted with roses on the runway at Tegel. Naturally, the numbers are dwindling now and the city authorities no longer charter flights. Last year, there were fewer than a hundred guests. Soon there won’t be any.

‘And then what?’ Martina Voigt, the curator of the exhibition, asks. ‘What will happen when there’s nobody left to tell this story?’ One could argue that question has already been answered, in part, by the 2016 state elections, when 14.1 per cent of Berliners voted for the far-right, anti-immigration party Alternative für Deutschland. This presents a problem for Martina. When former refugees return to the city of their youth, she wants them to see how different it is now. And usually, she says, they get a good impression of Berlin. In fact, she has ‘searched hard for any bad experiences and found none’. And yet she feels concerned. She shows us a poem written by one of the returning refugees. He praises the city that Berlin has become, how ‘nobody gives orders or barks’ any more. There’s a long silence and then her voice cracks as she speaks: ‘But he’s wrong. We just made sure he didn’t meet those people.’

Back home, I begin a search for the mythical document that only the most underemployed of my close relatives have managed to read: my great-grandfather’s memoir. It is comprised of nearly 2,000 pages of unpublished typescript, in German, which he started writing in 1961 while living in North Carolina. When I ask my parents about it, they are not sure where it is. Maybe in a chest of drawers in my grandmother’s cottage in the Scottish Borders? Maybe in the Jewish Museum in Berlin? Of my immediate family, the only person who has read it is my father. He is a historian, specialising in seventeenth-century Holland, and so when he says he found it ‘a bit of a slog’, I know I’m in trouble.

Siegfried wrote his memoir in the last nine years of his life and he was still adding footnotes to it when he died in 1970. The substantially abridged English translation, which is all I find at my grandmother’s cottage, was completed forty-two years later, by my great-uncle, aged ninety-two, while he was living in a retirement home. He died just days after finishing it. Holding this ring-bound document in both hands, I am aware that it was an end-of-life project for two generations of my family.

Across hundreds of thousands of words, Siegfried manages to say almost nothing about their first-hand experience of the Third Reich. The memoir ‘officially’ ends on the day Hitler came to power. Much of the book details our ancestral history, starting with Jizhak Merzbacher, an eighteenth-century trader of animal hides from north-east Bavaria. The manuscript also spends a great deal of time on Siegfried’s grandmother, Nanny Merzbacher, including a hundred pages of her diary which he faithfully typed up, word for word. In the English translation, her daily life was deemed insufficiently interesting and the diary was cut in its entirety, though we do still get a sense of Nanny’s character, how she ‘had a sharp tongue and left no good hair on anyone . . . including herself’, a quality that survived undiluted in my own grandmother, Dorothea.

When Siegfried does reflect on his life in Oranienburg, raising a family in the shadow of fascism, one of his frustrating tics is that he typically follows difficult episodes with soothing memories of a holiday. No sooner has he recalled a train journey to Berlin in early 1933, when he was ‘too cowardly’ to intervene as a storm trooper dragged a ‘polite and quiet’ Jewish-looking gentleman out of the carriage, than he is drawn back to 1893, aged ten, travelling with his family to the coast at Blankenberge where he saw – for the first time – the ‘infinite extension’ of the sea. If a single sentence could define the whole book, it would be: ‘From this unpleasant subject I now return to my childhood vacations.’

In July 1930, the yard behind Siegfried’s factory was stacked high with fragrant piles of prune and peach stones, coconut and almond shells, raw materials for the production of activated charcoal, a substance which could trap and neutralise poison gases. Siegfried had just accepted a promotion at Auer, moving from toothpaste to the role of supervisor in the ‘protection’ division. This meant he was in charge of producing and testing charcoal filters in gas masks. Day and night, his assistants fed the peach stones and coconut shells into a rotating furnace and, for the first time in his twenty-three-year-long career, Siegfried felt he was doing something valuable.

For the most part, testing the filters against poison gas could be done in the relative safety of the laboratory, with a step photometer and an exhaust hood. Nevertheless, it was sometimes deemed necessary for the chemists to undertake more dangerous field tests. In his first week, he’d watched a young colleague succumb to nitrogen narcosis. Wearing a full-face mask, Dr Fries had run in wild circles round the ice cellar before dropping to the flagstones. Siegfried thought he was dead but, when they pulled the mask off, he sat up as though waking from a nap.

As the division’s new supervisor, Siegfried felt there was pressure on him to show willing – which was why he found himself sitting on a bench in an airtight tank while three colleagues peered in through a window of inch-thick glass. The air he was breathing contained 4,000 parts of CO₂ per million, roughly ten times the density you would expect in the air outside, yet still far below the level of oxygen deprivation. Presuming he breathed normally, which he was trying to do, then he should feel no more uncomfortable than he would in a stuffy meeting room. He and his colleagues were also studying how humidity interacts with CO₂ and he could feel the wall-mounted humidifier breathing on his neck. He should have told his colleagues that he was prone to bouts of light-headedness – had fainted once while climbing the stairs to his apartment – but he didn’t want to seem feeble. He looked out at the faces of his new colleagues: Herr Otte, Herr Worbs and Dr Puetter. He could see their mouths moving but couldn’t hear them.

He opened the pad on his lap and, with his pencil, made notes in Gabelsberger, an elegant shorthand. The atmosphere in the tank was close and tasted faintly acidic but he felt fine.