In the quiet of the forest, Aleksander holds a rusted pistol and turns it over. Others gather round to admire the handgun, each feeling its weight before shooting an imaginary bullet into the trees. More detritus of war is placed on a picnic table – a swastika-adorned badge, shards of shrapnel, a Soviet medal inscribed “Proletarians of all countries, unite!” The remnants of fallen regimes.

The men are among the thousands of detectorists across eastern Europe hunting for relics of the Red Army, the Third Reich and Imperial Russia. Beneath ploughed field and remote woodland is buried treasure from a turbulent, vanishing past. Even today, the war dead lie in these lands. Sometimes bodies are found.

Is this grave robbing? I never take jewellery or gold teeth from the dead. But medals… that’s OK

Each weekend this group, calling themselves the Masovian Guardians of History, heads to the Polish backwoods to escape the city. There is a builder, barman, factory worker, office clerk, postman and sushi chef, among others. Fuelled by vodka and a passion for history, they strip old battlefields of artefacts while trying to dodge the police.

On a drizzly summer afternoon 24 hours earlier, I meet Aleksander in a station outside Warsaw. The 30-year-old, with cropped hair and an athletic frame, has just finished his shift at a railway parts factory. He gives a polite yet calibrated smile and leads me to his car.

As we drive across flat, open country, I try to imagine the bloodletting that passed on this, the former Eastern Front – the armoured clashes, the scorched-earth policies, the millions dead. But it is impossible. It’s not long until conversation turns to politics. “Immigration is not good,” says Aleksander, unprompted. “We should stop the mix of people and cultures.” What about his compatriots who face similar hostility in the UK? “They should not be in Britain in the first place,” replies the amateur kickboxer-turned-triathlete. “These Poles are exploited and should stay here to make our country great.”

The rising storm: German troops parade in front of Hitler and his Nazi generals after entering Warsaw on 5 October 1939. Photograph: Keystone/Getty Images

The countryside grows emptier and, beyond the nondescript town of Międzyrzec Podlaski, we reach a small dwelling with rundown outhouses, guarded by a yapping mongrel. A middle-aged man dressed in camouflage trousers and black T-shirt opens the gate. A few detectorists stand around, drinking beer. Several more soon pitch up, dressed in the outfit of choice – khaki fatigues.

Suddenly, someone lets out a cheer. Dariusz has arrived. In sweat-stained army surplus, he swaggers over – metal detector slung over his shoulder, paunch protruding from his vest. A silver cross hangs on a heavy chain around his neck. His right arm is tattooed with a Winged Hussar of the old Polish cavalry. After greeting his friends, he challenges me to a fight but gets distracted by our photographer – a Mexican. “Ah, you’re from the Zetas cartel! You are my boss!” As a joke, he holds two knives to the throat of our translator then heads off for another beer, tripping down the steps to the house. “Ha!” he guffaws. “Instant karma!”

The dig will start in the morning. The plan now is to get plastered. Lager flows among innumerable shots of vodka flavoured with cherry, plum, quince or walnut. Dariusz prepares a campfire for a pot that bubbles with potatoes, onions and chunks of pork. This hearty dinner fails to soak up all the booze and, by the early hours, the detectorists pass out. Three collapse fully clothed on to a single bed.

Lost and found: guns, coins and bullet shells found by the ‘Masovian Guardians of History’ while metal detecting. Photograph: Antonio Olmos/The Observer

Two hundred miles away lies Kraków, the base of DCI Bartłomiej Morek. This policeman runs the Vinci unit, targeting unaccredited detectorists, art thieves, forgers and relic smugglers. “We don’t want to fight hobbyists, but my job is to protect archaeological sites,” he says. “We must stop detectorists destroying them.”

Finds are often flogged on the black market, with online bazaars distributing artefacts to countries around the world, including the UK. “We have tools to locate people on the other side of the internet,” says Morek. “Once we know an item is illegal and inside Poland, there’s no problem securing a search warrant.” Landing a prosecution is harder. “Detectorists know how to escape a conviction and share their knowledge about how to stay free.”

My father killed himself when I was 19. After that I drank hard for three years – a litre of vodka a day. Now I’m better

Metal detecting without the right permit can incur a £1,000 fine or a month in prison. Those guilty of possessing salvaged weapons and explosives – even if rusted or deactivated – may be jailed for several years, though sentences are often suspended. Critics say Polish law on historical objects is ambiguous, leading to arbitrary punishment and offering no financial incentive to hand in artefacts.

Back near the Poland-Belarus border, as the Masovian Guardians of History gather on Saturday morning, Dariusz is jovial despite last night’s heavy session. His fascination with the past was kindled by his grandfather, a Second World War veteran captured by the Nazis and used for slave labour. “I am surrounded by history,” says Dariusz, 35, in broken English. He graduated with a history degree and now, after a turbulent 20s, works as an itinerant farmhand, picking fruit in southwest Germany.

“My mother was a civil servant, my father committed suicide when I was 19. After his death, I drank hard for three years. One litre of vodka per day sometimes. I was very aggressive and fighting in the streets, so my girlfriend said: ‘You have a problem, go see psychologist.’ One day, I just wake up and decide to stop. Now I’m better. Sometimes I see my father in me, but I make a choice and fix it.”

‘When I am digging, I am escaping’: Piotr scours a field for buried treasure of the wars past. Photograph: Antonio Olmos/The Observer

As with most here, Dariusz is a nationalist. His right-wing views extend to Islam (“I get my machete for all the Muslims”), gay rights (“This is a Catholic country – I think most people don’t like the gays”) and recent demonstrations against the government’s controversial plan to take control of the Supreme Court (“This protest is sponsored by Soros and run by old communists”). On American politics, he is nonplussed (“Obama or Trump – same shit”).

Despite his views, Dariusz is affable, all bluster. As a fellow detectorist explains: “He’s the most popular guy here. He has a big heart. If he saw a Muslim woman in trouble, he’d be the first to help.” He’s still single. “Girls don’t like it when I’m away digging,” he says. “Maybe I have wife one day, but now digging is winning.”

Over breakfast, the group knocks back coffees and beers and then, dressed in combat gear, piles into a van for the bumpy ride into the mosquito-infested woods. This area was once scarred by a series of clashes – between the Wehrmacht and the Red Army in 1944, between the Austro-Hungarian and Russian empires during the First World War, between Polish rebels and the tsar in 1831 – along with assaults during the 17th-century from Russia, Sweden and Zaporozhian Cossacks.

No one here bothers with the bureaucratic rigmarole of obtaining accreditation to dig. One of the regulars, a policeman, is usually on hand to help them evade the law. “Once we got stopped by two cops,” says one detectorist. “But he was more senior so they let us go.”

Before the dig, the group gathers round a recently salvaged collection of weapons from the battlefield – rifles of Imperial Russia, semi-automatic Mausers, a pair of Ruby and Tokarev pistols – along with rare medals, coins and military insignia. Then they fan out into the green glow of the forest. Their instruments buzz, bleep and hum as they head through stands of birch and maple, oak, pine and spruce. One of them stumbles across a wartime trench, already ransacked.

I spot Aleksander, his sensor flicking from side to side. He and his grandfather – a survivor of massacres carried out by Ukrainian nationalists in 1943 – enjoyed long conversations about history, which nurtured his passion for the past. There is another reason for his love of metal detecting. “My factory is stressful and dirty, but the forest is clean,” says Aleksander. “I rest my brain and escape from reality.”

‘Archaeologists know how to do a proper excavation. We don’t just tear objects from the soil’: Dr Krzysztof Jakubiak (left) and Dr Tomasz Nowakiewicz from Warsaw University. Photograph: Antonio Olmos/The Observer

But his pursuits have created a family rift. His sister, an archaeologist, does not approve: “We don’t talk much any more.” Their relationship is symptomatic of the tensions between detectorists and archaeologists, about whom Aleksander is scathing: “They just attend conferences and spend money. They’re the real thieves.”

The feeling is mutual. Dr Tomasz Nowakiewicz, from the University of Warsaw’s Institute of Archaeology, says: “We’re no longer in the 19th century. Calling yourself a collector is no excuse.” Institute director Dr Krzysztof Jakubiak tells me the detectorists’ slapdash methods destroy an object’s context – critical for interpreting past human activity. “We know how to do proper excavation and extraction. We don’t just tear objects out from the soil. Within 10 years, we risk losing all these objects.”

The detectorists scour the forest and find medals, coins and shrapnel, but not much more. Sweaty, bug-bitten and ready to refuel, they stop at a pizza parlour. Artur, a factory worker, orders a carafe of vodka and passes out before we arrive back at the rented house.

They consider themselves custodians of history. “For me, these objects are priceless,” says one. “They connect me to the past. I don’t want to leave them to rust. This is our national history, but they’re my artefacts.”

That night there is axe-throwing and vodka shots while Marek, a sushi chef, plays the same patriotic military tune on repeat. After too much boozing, he is carried to bed, but by breakfast the next morning he has already necked three beers.

The group walk through an orchard and cross a marsh amid birdsong and the buzz of insects. It is bucolic, another world from the one that Jan Demczuk remembers from his childhood. We meet the 90-year-old, retired farmer chopping firewood outside his barn. He doesn’t seem bothered by the khaki-clad strangers probing his land for trinkets and trophies.

“There was fighting around here in September 1939 between Germans and Poles,” recalls the old man. The frontline returned in summer 1944 as the Soviets pushed back against the Wehrmacht. “I remember soldiers running past. There were many explosions.”

As battle raged, Demczuk describes his mother taking him with their piglets to hide in a field. The sow, missing her litter, escaped from her pen and joined them, as the terrified young boy cradled the tiny animals. Asked about life under occupation, he answers: “As hosts, the Nazis were perfect. There was order. Sure, they killed innocent people, but it was war.”

Then and now: chatting with Jan Demczuk, a farmer who lived through the Second World War. Photograph: Antonio Olmos/The Observer

His recollections do not capture the horror that unfolded in this area, home to a large, centuries-old Jewish community. A ghetto was established, then came the mass deportations by cattle wagon to extermination camps. Less than 1% of the local Jewish population is believed to have survived.

The detectorists survey a meadow then move to a raspberry plantation, occasionally crouching down to dig up the goods. Dariusz is unconcerned about unexploded ordnance. “No problem. It’s not dangerous. If it blows up, my brain is dust and I don’t even know.”

Like Aleksander, an obsession with past heroics is not his only motivation for rummaging in the dirt. “When I am digging, I am escaping,” he explains. “I just forget.”

There is a beep and Marek and Piotr begin excavating with great purpose. They hollow out a few inches then test the soil with a detector. Another beep. Piotr grabs his spade and digs out clumps of earth. He reaches in, clasps his fingers around the object and lifts it out. A helmet, perhaps? An SS officer’s Luger? It’s a rusted horseshoe. He tosses it away.

Bottle tops and farm junk may account for most finds but, very occasionally, something extraordinary is unearthed. Back at the house, a builder called Sebastian is knocking back a late-morning beer. This father-of-two in his late 30s is going through a divorce and has come to the countryside to clear his head.

He describes how, several years ago, he went looking for militaria near Bohukały, a village by the Belarus border. German Panzer divisions had rumbled through here on their lightning advance eastwards in 1941. Three years later, this tiny village was again at the centre of fierce fighting during the Soviets’ counter-offensive. Sebastian heard that following the battle locals had buried the dead in bomb craters. He set out across this necropolis and his detector soon started beeping. He dug up a flask then burrowed further and found a dog tag, and then a skull. The more he dug, the more bones he found, finally exhuming 26 skeletons.

“I was excited and emotional,” says Sebastian. “It was intense.” He took the dog tags, covered the bones and informed a German organisation that repatriates the remains of missing soldiers. “It doesn’t matter if they’re Polish, Russian or German. They’re soldiers with loved ones who don’t know what happened.” He also pocketed spent ammunition, shell fragments and a medal awarded to those wounded in action. Is this grave robbing? “I never take jewellery or gold teeth from the dead,” he replies. “But medals – that’s OK.”

The group gathers for one last beer, inspecting the weekend’s haul. Besides shrapnel and cartridge cases, there are coins and military buttons from the 19th century, a 1660s shilling, a bronze medallion from the Russian-Turkish war (1877-78), a Nazi map-reading instrument, and a handsome piece of silver. But they’ve not made that dynamite discovery that could earn a fortune and generate a legend. “No problem,” smiles Dariusz, shrugging his shoulders. “We try again next weekend.”

Some names have been changed