Of the estimated 70 million people killed in World War Two, 26 million died on the Eastern front - and up to four million of them are still officially considered missing in action. But volunteers are now searching the former battlefields for the soldiers' remains, determined to give them a proper burial - and a name.

Olga Ivshina walks slowly and carefully through the pine trees, the beeps of her metal detector punctuating the quiet of the forest. "They are not buried very deep," she says.

"Sometimes we find them just beneath the moss and a few layers of fallen leaves. They are still lying where they fell. The soldiers are waiting for us - waiting for the chance to finally go home."

Nearby, Marina Koutchinskaya is on her knees searching in the mud. For the past 12 years she has spent most of her holidays like this, far away from home, her maternity clothes business, and her young son.

"Every spring, summer and autumn I get this strange sort of yearning inside me to go and look for the soldiers," she says. "My heart pulls me to do this work."

They are part of a group called Exploration who have travelled for 24 hours in a cramped army truck to get to this forest near St Petersburg. Conditions are basic - they camp in the woods - and some days they have to wade waist-deep through mud to find the bodies of the fallen. The work can be dangerous, too. Soldiers are regularly discovered with their grenades still in their backpacks and artillery shells can be seen sticking out of the trees. Diggers from other groups elsewhere in Russia have lost their lives.

Marina holds up an object she has found, it looks like a bar of soap, but it is actually TNT. "Near a naked flame it's still dangerous, even though it has been lying in the ground for 70 years," she says.

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Many countries were scarred by World War Two, but none suffered as many losses as the Soviet Union.

On 22 June 1941, Hitler launched Operation Barbarossa, the largest and bloodiest campaign in military history, aimed at annexing vast areas of the USSR to the Third Reich. St Petersburg, then known as Leningrad, was one of his main targets. In less than three months, the advancing German army had encircled the city and started pounding it from the air.

'They were killing us like flies' Image copyright Dean Arnett Mikhail Zorin cried when he first saw the gun he was expected to use to repel the German invasion. "It was so big and heavy," he says. "I was scared, how was I supposed to shoot?" It was an antiquated model from the Tsarist days - and shortages meant they were only given one between two. The 18-year-old Zorin endured nine hellish days fighting in Nevsky Pyatachok - the tiny strip of land by the Neva River where 260,000 Soviet soldiers died trying to break the siege of Leningrad. "They were killing us like flies," says the 90-year-old. "There were corpses lying all over the place. The ground was mashed up by bombs and shells and we simply weren't equipped to fight back."

But attempts to take the city by storm fell through, so Hitler decided to starve it into surrender. For more than two years, the Red Army fought desperately to cut through German lines.

Olga and Marina are working near the town of Lyuban, 80km (50 miles) south of St Petersburg. Here, in an area of just 10 sq km, an estimated 19,000 Soviet soldiers were killed in just a few days in 1942. So far the diggers have found 2,000 bodies.

Ilya Prokoviev, the most experienced of the Exploration team, is carefully poking the ground with a long metal spike. A former army officer with a droopy blonde moustache, he found his first soldier 30 years ago while walking in the countryside.

"I was crossing a swamp when suddenly I saw some boots sticking out of the mud," he says.

"A bit further away, I found a Soviet helmet. Then I scraped away some moss and saw a soldier. I was shocked. It was 1983, I was 40km from Leningrad and there lay the remains of a soldier who hadn't been buried. After that there were more and more and more, and we realised these bodies were to be found everywhere - and on a massive scale."

There was little time in the heat of battle to bury the dead, says Valery Kudinsky, the defence ministry official responsible for war graves.

"In just three months the German death machine covered more than 2,000km (1,250 miles) of our land. So many Red Army units were killed, wiped out or surrounded - how could anyone think about burials, let alone records of burials, in such conditions?"

Immediately after the war, the priority was to rebuild a shattered country, he says. But that does not explain why later the battlefields weren't cleared and the fallen soldiers not identified and buried.

The diggers now believe that some were deliberately concealed. The governing council of the USSR issued decrees in 1963 about destroying any traces of war, says Ilya.

"If you take a map showing where battles took place, then see where all the new forest plantations and building projects were located, you'll find they coincide with the front line. Nobody will convince me they planted trees for ecological reasons."

Image copyright Dean Arnett

If you crouch down in the woods near Lyuban, a series of grooves in the earth can be clearly made out.

"They actively planted new trees on the battlefield - they ploughed furrows and put the trees exactly in the places where the unburied soldiers were lying," Marina says.

She recently unearthed a helmet and in order to find its owner, the team had to uproot two nearby trees.

'He promised he would come back' Image copyright Other I will never forget the story of 21-year-old Khasan Batyrshin, writes Olga Ivshina. Khasan went missing in 1943. His family contacted the government every year asking for information about him, but the answer was always the same. "No data found. Soldier missing in action." But the Batyrshins never gave up. Even at the age of 105 his mother repeated: "He will come back. He promised me. He will even move mountains." We found Khasan last year near Nevskaya Dubrovka. The bodies of those killed there were just put into pits made by falling shells. It was one of the last days at the dig. Everyone was tired, but Khasan inspired us. We found his ID tag. It is always a miracle, because many soldiers didn't have them at all. And even those who did often didn't fill them in. The piece of paper in his tag was neatly pencilled in. It was not easy to decipher, but after four hours, Khasan got his name back. When his family got the news, the first thing they did was visit the tomb of Khasan's mother to tell her. She had died a year earlier.

"When we cleaned away some clumps of earth from the roots we saw two hands tangled up in them. Then we found a pelvis and some ribs between the roots. So we think the whole soldier was underneath the roots and the trees were growing on top of him."

But how could anyone - farmers or workmen - get on a tractor and plough over land littered with human remains?

"If they refused to plough a field because there were corpses or bones in it, they'd just be sacked," says Ilya. "If you lost your job in those days you were a non-person - you didn't exist. That's what life was like in the Soviet Union." Plus, it was less than two decades after the war. The workers had endured far worse horrors, he says.

There are horrors for the diggers, too.

Nevskaya Dubrovka, on the banks of the River Neva, was the scene of one of the bloodiest campaigns of the Leningrad siege. The Red Army fought tooth and nail to secure a narrow stretch of river bank in an attempt to break the blockade. Hundreds of thousands of troops, used as little more than cannon fodder, were slaughtered.

Diggers discovered a mass grave in the area last summer. The soldiers may have been thrown into the pit by their comrades or local villagers as a hasty form of burial, or even by the German Army, anxious to prevent an epidemic among its troops.

"There must have been 30 or 40 soldiers in there. Four layers of people one on top of the other," says Olga, as she sits by the campfire. "But the skeletons were all mixed up and smashed. Here you have a head - there a leg…" She pauses and stares into the fire. "Once you've seen that, you'll never forget it. You are no longer the same person you were before."

Going back to city life and her job with the BBC Russian Service is sometimes hard after a few weeks in the forest. When her friends in Moscow complain about not being able to afford a good enough car or designer clothes, she feels alienated.

"Everything seems so pointless - even my job as a journalist - and sometimes I think, 'What am I doing?' But here, on the dig, I feel we are doing something which is needed."

For Olga - who sang hymns to Communism in her primary school, then learnt about profit and loss at secondary school - volunteering as a digger also provides a moral compass in confusing times.

"Sometimes you need to know that you are doing something which is important, that you are not just a piece of dust in this universe. This work connects us to our past. It's like an anchor which helps us to stay in place even during a storm."

Finding the dead is only one part of their mission. Rescuing them from anonymity is the other.

In Moscow an eternal flame burns at the Tomb of the Unknown Soldier in the shadow of the Kremlin Wall, but for the diggers, the best way to honour those who lost their lives is to give them back their identities.

"The soldier had a family, he had children, he fell in love," says Ilya. "Being unknown is nothing to be proud of. We are the ones who made him unknown."

Image copyright Dean Arnett Image copyright Dean Arnett Image copyright Maxim Lomakin Image copyright Dean Arnett previous slide next slide

But discovering who they were is not always easy, especially after so much time has passed.

"The more data we can collect from the spot, the better the chance we have to identify a soldier," says Alexander Konoplov, the leader of the Exploration group. Sometimes they find old coins with the soldiers, given to them by their families. The belief was that if the family lent him a few coins, he would come home to repay the loan.

But while personal items can build up a picture of the person, they can't help find his name, or place of birth. Initials scratched into spoons and bowls are good. But the key is usually an ID tag.

Media playback is unsupported on your device Media caption Olga Ivshina reports on a chance discovery

During World War Two, Soviet soldiers' ID tags were not made of metal - they were small ebony capsules containing a small piece of paper for their personal details. Sadly, the papers are often illegible. Others were left blank because many soldiers were superstitious - they believed filling in the forms would lead to certain death.

Alexander, who ran his own business selling food products before becoming a full-time digger, is holding a bullet case plugged with a small piece of wood. He hopes that it is an improvised ID tag. But when he turns it upside down in his hand, what comes out of it is not a roll of paper, but a trickle of brown liquid.

"Sometimes we find messages with the soldier's name," says Alexander. "Some wrote, 'If I am killed, please pass this on to my girlfriend or my mum.' You can't help feeling touched by it."

Exploration is one of 600 groups of diggers from all over Russia who have found and reburied a total of 500,000 soldiers so far.

These teams are known as the "white diggers", but there are also those dubbed "black diggers" who search for medals, guns, coins or even gold teeth which they sell online or to specialist dealers. They are not interested in identifying the soldiers - they just leave the bones in the ground.

Alexander has a strict set of guidelines about how the remains should be excavated, labelled and stored. Each soldier is photographed and their location is recorded and entered into a digital database.

Image copyright Various Image caption Ebony ID tags; a volunteer tries to decipher their contents; a grenade found in the forest

Image copyright Various Image caption Diggers Marina, Ilya and Olga

If a decades-old ID tag cannot be deciphered by the team on the ground, it is carefully packed and sent to the team's headquarters in the Volga city of Kazan.

The team's technician, Rafik Salakhiev, uses ultraviolet light and digital imaging to reveal the faded pencil marks. "Let's try to enhance purple colours on this yellow paper," he says. "We can reduce the saturation and yes! We start to see some letters…"

Once a name emerges, the diggers use old army lists, classified documents and contacts in the military or police to identify the soldier precisely and to locate surviving members of his family.

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"Every new search gets to me as if it was the first one," says Rafik. Many of the relatives are now elderly and may not be in good health. "When you call the relatives, before telling them the news, you try to prepare them. Even if they have been waiting for a long time."

But tracing a soldier's family can take years - on occasions more than a decade - especially if the family moved after the war.

When, in 1942, people in First Lt Kustov's home village heard he was missing, they suspected him of deserting and collaborating with the Germans. They branded his young son and daughter traitor's children and the family were forced to leave. It took Ilya Prokoviev months to track them down.

"When we told them that we had found their father's remains, for them the feeling was just indescribable. They knew that he hadn't just deserted, that he couldn't have behaved like that, but there was never any proof until 60 years later."

From the archives, the diggers worked out that Kustov had been the commander of one of Stalin's notorious shtrafbats, a battalion made up of prisoners and deserters. Only a trusted officer and staunch communist would have been appointed to such a post.

"They had managed to restore historical truth and honour their father's memory," says Ilya. "It was the main event of their lives, I think." Kustov's children took his remains and buried them next to their mother, who had waited her whole life for her husband to return.

Image copyright Dean Arnett

Near the banks of the River Neva, close to the mass grave found by diggers, a Russian Orthodox priest chants prayers as he walks around the rows of bright red coffins laid out on the grass.

The children, grand-children and great-grand-children of the soldiers they unearthed look on, some quietly sobbing.

Valentina Aliyeva is here to bury the father she has not seen since she was four years old. For seven decades, the only link she had with him was a black and white photo of their former family home.

"My mother remarried some years later and everyone told me to call my stepfather Daddy. But I refused - I knew who my real dad was," she says, her eyes filling with tears. "What those diggers have achieved means so much to me. I can't tell you how grateful I am."

Tatiana Uzarevich and Lyudmila Marinkina, twin sisters in their early 50s, have travelled from the remote region of Kamchatka - nine hours away by plane. The diggers found their grandfather's ID tag in the mass grave. When they were unable to trace his family, the group put out an appeal on the evening news.

The twins' elderly mother was stunned when she heard his name - Alexander Golik - the family had searched in vain for years. His disappearance had left his wife and children destitute. "The fact that he was missing in action meant that my grandmother was not entitled to any of the financial support given to other relatives after the Great Patriotic War. She didn't get a penny and she had four children to raise," says Lyudmila.

"My mum was so hungry all the time, she begged the other kids for pieces of bread at school.

"She only remembers the shape of her fathers' hands - but she had memories of a kind, good man," says Tatiana. "We just had to come to this reburial service to visit the place where he died and accompany him to his final resting place."

Image copyright Dean Arnett

Image copyright Dean Arnett

Image copyright Dean Arnett

The walls of the large, newly dug grave are draped with red cloth - an act of respect normally accorded only to army generals. Young men dressed in Soviet-style army uniforms form a guard of honour. Visibly moved, as coffin after coffin is carried past to be buried, some of them look up to the sky. There is a belief that birds flying overhead transport the souls of the dead.

There are more than 100 coffins - each contains the bones of 12 to 15 men. The diggers would like each soldier to have his own, but they can't afford the extra 1,500 they would need for today's service.

This is the culmination of months of work by the volunteers. It's what it's all for - bringing a semblance of order to the moral chaos of the past, and paying tribute to those who gave their lives.

In the spring they will resume their searches in the forests and fields where so many were slaughtered. They are determined to continue until the last man is found. But it could be a life's work - or more.

"There are so many unburied soldiers, it will take decades to find them. There will definitely be work for our grandchildren," says Marina. "But nature is working against us. The remains are decomposing and it is getting harder to find the bones, ID tags and army kit." The more years that go by. The less information there is.

"We need to continue to do this for ourselves, so our souls can be at peace," says Ilya. "It has become the meaning of our lives."

Image copyright Dean Arnett

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Website production by Catherine Wynne. Images courtesy of Dean Arnett, Lucy Ash, Olga Ivshina and Maxim Lomakin