Archaeologists are beginning the most detailed ever study of a Western Front battlefield, an untouched site where 28 British tunnellers lie entombed after dying during brutal underground warfare. For WWI historians, it's the "holy grail".

When military historian Jeremy Banning stepped on to a patch of rough scrubland in northern France four months ago, the hairs on the back of his neck stood up.

The privately owned land in the sleepy rural village of La Boisselle had been practically untouched since fighting ceased in 1918, remaining one of the most poignant sites of the Battle of the Somme.

In his hand was a selection of grainy photographs of some of the British tunnellers killed in bloody subterranean battles there, and who lay permanently entombed directly under his feet.

When most people think of WWI, they think of trench warfare interrupted by occasional offensives, with men charging between the lines. But with the static nature of the war, military mining played a big part in the tactics on both sides.

The idea of digging underneath fortifications in order to undermine them goes back to classical times at least. But the use of high explosive in WWI gave it a new dimension.

One of the most notable episodes was at the Battle of Messines in 1917 where 455 tons of explosive placed in 21 tunnels that had taken more than a year to prepare created a huge explosion that killed an estimated 10,000 Germans.

Tunnelling was mainly done by professional miners, sent from the collieries of Britain to the Western Front.

La Boisselle: A village under siege 28 Sep 1914 - German advance on Amiens halted by French forces. Fierce fighting over the cemetery and farm buildings

- German advance on Amiens halted by French forces. Fierce fighting over the cemetery and farm buildings Dec 1914 - French begin mining to retake the farm. Intense struggle above and below ground

- French begin mining to retake the farm. Intense struggle above and below ground Aug 1915 - British take over the sector from the French with tunnels now at a depth of 40ft (12m)

- British take over the sector from the French with tunnels now at a depth of 40ft (12m) 1 July 1916 - British launch disastrous Battle of Somme with village on main axis of attack. Two huge mines - Y Sap and Lochnagar - create massive craters, one 270ft (82m) wide by 70ft (21m) deep

- British launch disastrous Battle of Somme with village on main axis of attack. Two huge mines - Y Sap and Lochnagar - create massive craters, one 270ft (82m) wide by 70ft (21m) deep 4 July 1916 - British capture village after further heavy fighting

- British capture village after further heavy fighting March 1918 - German troops overrun trenches in the village during Operation Michael, part of the huge Kaiserschlacht offensive

German troops overrun trenches in the village during Operation Michael, part of the huge Kaiserschlacht offensive Aug 1918 - Welsh troops liberate La Boisselle

What happened at La Boisselle in 1915-16 is a classic example of mining and counter-mining, with both sides struggling desperately to locate and destroy each other's tunnels.

"When you stand on a spot and can look at a picture of a man still down there below you, it's amazing," Banning says.

"It just does something very strange to you, it makes the hairs stand up on the back of your neck."

After six years of painstaking paper research by fellow historian Simon Jones, the researchers had built up detailed knowledge of the individual tragedies involved.

They knew the exact locations and depths at which each man was lost, the circumstances of their deaths, and almost all of their names.

And yet it was only when the owner of the site chose to open it up to research that they were able to finally connect the stories to the place.

The Lejeune family, who have owned the land since the 1920s, have a deep affinity with the site and have known many British veterans who served at La Boisselle.

But it was only after visiting the team's excavations at nearby Mametz last May that they decided to offer their land up for historical study.

Archaeologists, historians and their French and German partners now aim to preserve the area - named the Glory Hole by British troops - as a permanent memorial to the fallen.

Digging does not start until next year, but the first practical steps of mapping the tunnels and trenches using ground-penetrating radar, and exploring the geophysics are under way.

Some open tunnel sections have already been entered and are considered remarkably well preserved.

The team intends to leave the bodies undisturbed in the collapsed tunnels, but any others found in trenches will be reburied in accordance with the Commonwealth War Graves Commission.

Bomb disposal experts will be on standby to negotiate the unexploded ordnance they will inevitably uncover.

Image caption Mining operations were often part of ground offensives

They also expect to find graffiti on the walls, poetry, bottles of drink, and all manner of artefacts untouched since the day fighting ceased. In short, they say, it's a time capsule.

The long-term intention is to open the site to the public, and the whole project is expected to take five to 10 years.

For Jones, a former curator at the Royal Engineers Museum, the dig is about completing the stories of the two Tunnelling Companies (179th and 185th) who worked at the Glory Hole.

"Finding out about these men has become an obsession, and although we know a great deal about the lives of soldiers in WWI, these men have left very few clues as to their experience or feelings," he says.

Mining was perilous work in a hidden war, which remained a state secret for many years, meaning the men did not get the recognition they deserved.

By studying war diaries, tunnel plans, letters, maps and records, Mr Jones has identified 25 of the 28 British and all 10 French tunnellers at the Glory Hole. The number of Germans remains unclear.

Image caption Chris Lane, pictured inset alongside his great grandfather, says "it's important to know your past"

The British were lost between August 1915 and April 1916, sometimes individually but more often two or more at a time.

"Often men from the same pits preferred to work alongside one another and hence were lost together," Mr Jones says.

One such miner was Sapper John Lane, 45, from Tipton in Staffordshire, a married father-of-four who left his colliery for the Western Front with four colleagues. None returned.

On 22 November 1915, he and four others were killed 80ft (24m) below when a German mine exploded, in turn detonating a British charge of 5,900lb (2,700kg).

For his great-grandson Chris Lane, 45, from Redditch in Worcestershire, piecing together his relative's story has been a fascinating process.

He says they knew he was killed in a mine, but prior to his research, his grandfather always thought it was in Ypres in Belgium.

"It's important to know your past, one small incident for one family is history for lots of other people," he says.

The new dig is only the second on the Western Front to be officially sanctioned by the French authorities.

Patches of untouched virgin battlefield are rare. Most have been ploughed over, cleared or developed, and private landowners have been reluctant to hand them over for research.

It's a site of huge strategic importance. When the British launched the bloody Battle of the Somme on 1 July 1916, La Boisselle stood on the main axis of the attack.

Of the 1.5m total casualties in the four-month campaign, 420,000 British soldiers were killed, wounded or missing having gained just two miles - a loss of two men per centimetre.

Fellow historian Peter Barton says La Boisselle is the "holy grail" for historians, containing the "complete evolution" of trench warfare.

Media playback is unsupported on your device Media caption Military historian Simon Jones explains the importance of the excavation of the Glory Hole

"The site has got both sides of the line and the fourth dimension of underground warfare, making it a truly holistic project," he says.

"These are not just holes in the ground, they're homes - that was where you lived when you were holding the line.

"You became troglodytes. They designed, evolved and engineered a way of living and surviving, and had to go deeper and deeper as the shelling became more effective."

Barton's research took him to Munich and Stuttgart, where interpreters and translators have helped paint an even bigger picture.

"We'll know the Germans who killed the British and French, and vice-versa - it's the most supremely researched piece of battlefield on the Western Front," he says.

"Connecting those men who suffered and gave their lives there with their present day relatives is probably the most meaningful part."