Secret Tunnel Warfare

Secret Tunnel Warfare

PBS Airdate: January 6, 2016

NARRATOR: Almost a century since the outbreak of World War I, archaeologists in Europe are uncovering a unique time capsule from that conflict. For eight months, NOVA has been in Belgium, following a massive archaeological dig, and it's revealing how World War I became a technological arms race.

For four years, 70-million men fought in the first ever industrialized war. Nine-million died, as new and more powerful weapons enabled both sides to fight each other to a bloody standstill in trenches, many of them stretching across France and Belgium. It was a war that came to epitomize man's inhumanity to man.

SIMON VERDEGEM (Archaeologist): It must be a gas shell.

DAVID WHITHORN (Military Historian): Phosgene is not an instant gas. It can take 24 hours to come into effect, a horrible, horrible and painful and agonizing way to die.

NARRATOR: The dig is on a site where the entire history of that war is uniquely captured. We discover how the conflict went underground to blast the war out of its trench-bound deadlock.

JOHAN VANDEWALLE (Military Expert): Stop, stop, stop. Is it possible to turn the camera?

NARRATOR: It's here that Allied troops would secretly tunnel beneath the enemy lines and create the biggest explosion the world had ever seen.

With time and the weather against them, surrounded by unexploded ammunition, can archaeologists tell the story of this Secret Tunnel Warfare? Right now, on NOVA.

Messines: today, a peaceful town in Flanders, a region of Belgium on the border of France, but almost a hundred years ago, the site of the biggest explosion the world had ever seen. During World War I, this was the Western Front, where German and Allied soldiers faced each other in a hellish killing field. For over three years, the battle lines barely moved, as the war took lives by the hundreds of thousands.

The stalemate drove each side to develop increasingly deadly weaponry, triggering a high-speed, technological arms race that gave birth to modern mechanized warfare.

Now, prompted by a project to lay a two-kilometer pipeline around the town of Messines, a team of archaeologists has been called in to clear a path through the remains of these long-buried trenches, where so many fought and died. What they are discovering, buried in this unique site, is a snapshot of the first modern arms race.

ARCHAEOLOGIST: The round one, here, is German, and the pointed one is British.

NARRATOR: Miraculously preserved for a century, these trenches hold the entire technological history of World War I, from horses to tanks.

Racing against the construction schedule, they hope to preserve invaluable clues and to discover which technological innovations helped to finally break the deadlock.

Just outside Messines, work has begun on the new pipeline. The undercarriages of the heavy equipment are fitted with armor plating, and the glass has been re-enforced, because, nearly a hundred years after the war, the ground here is still full of unexploded shells.

It is rich, if dangerous, pickings for Belgian scientist Simon Verdegem, who leads the archaeological dig. Working alongside him are ex-Belgian military personnel trained in bomb disposal.

But one of their first discoveries offers evidence of the decidedly low-tech beginnings of the conflict.

SIMON VERDEGEM: It looks like the jaw of a horse. You can see the jaw, the end of the jaw. Here you have the teeth. This is the front. There are some more teeth over here…came loose; that one belongs here. It's definitely from a horse.

NARRATOR: World War I began in August, 1914, when Germany invaded Belgium and France. The Allies pushed the Germans back, eventually digging in at Messines and elsewhere. At the beginning of the war, the British army had 165,000 horses on its books, and in the slaughter that followed, it was not only the soldiers who would die.

SIMON VERDEGEM: You can see a row of teeth.

NARRATOR: Horses and mules were used extensively in the war, from pulling artillery to carrying water to the troops at the front. They were the warhorses.

At the start of the conflict, mounted cavalry were expected to lead the fight. It was a British cavalry unit, the 9th Lancers, who were holding part of the town of Messines, in the autumn of 1914. The Germans who were approaching the town were intent on removing them.

ALEXANDRA CHURCHILL (Historical Researcher): The Lancers are in trenches to the east of Messines, and, prior to the German attack, they are being heavily shelled. It's quite a miserable position for them to be in, because not only have they got to think about protecting themselves, but they have got horses to look after as well. It's effectively like dragging a small child around the Western Front, because you have constantly got to give care and attention to the animal; at the same time, you are still expected to do everything an infantry battalion is doing.

NARRATOR: Among the officers of the 9th Lancers, defending the town, were two brothers in their early 20s, Douglas and Lenny Harvey.

At dawn, on the 31st of October, the Germans launched their push for Messines.

ALEXANDRA CHURCHILL: And that day, for the 9th Lancers, is just horrific. The Germans successfully push the Lancers back and force them to retreat back into the town itself. And they are getting fired on from the front to the side from machine guns rifles and artillery shells, so it's a pretty dire situation.

NARRATOR: The 9th Lancers re-grouped at the cemetery, before being pushed from the town, which then fell to the Germans.

The battle had claimed the lives of both Douglas and Lenny Harvey. The war was barely three months old.

Having taken Messines, the Germans now set about consolidating their gains by digging trenches to protect their troops. The front line was etched into the battlefield.

Only days into the dig, archaeologists uncover the floor of a World War I German trench, part of a huge network which defined the war.

PAUL REED (Military Historian): Both sides dug in. They dug trenches to protect their troops. The Germans dug in at Messines, and the British just below, separated by a short stretch of no-man's land that could be just 10 yards apart. And by early 1915, this massive network of trenches now stretched from the North Sea coast right through and down to the Swiss border: 450 miles of continuous trenches.

NARRATOR: Both sides tried, with brute force, to break this deadlock on the Western Front. When British troops went over the top of the trenches at the Battle of the Somme, they suffered nearly 60,000 casualties on the first day, alone. At the battle of Verdun, over 700,000 fell, over 10 months of fighting. But, for the time being, nothing defeated the trenches.

At Messines, the team has been struggling on the site. A day's worth of rain has transformed the dig into a quagmire. It's a scene that would be all too familiar to the soldiers of the First World War. The archaeologists bail out the trenches with their safety helmets, as soldiers would have done with their steel helmets.

ALEXANDRA CHURCHILL: In the winter of 1914–1915, if it wasn't raining, it was snowing, so, consequently, in the trenches, it was muddy beyond belief. They were sinking up to their knees and some even deeper trying to get through them. It got so bad that they had to abandon trenches, and men were chancing going over the top and risking sniper fire to be able to not suffer the discomfort of getting covered in this mud and having to wade knee deep through it.

PAUL REED: If you look at the map, just ahead of us, probably on the rise…

NARRATOR: Helping the archaeologists, at the Messines dig, is the discovery of a highly detailed German map of the trench systems.

Marked on the trench map, in front of both the German and British positions, are belts of barbed wire. Barbed wire was an unexpectedly effective and ferocious barrier, feared and hated by the soldiers who had to get through it.

SIMON VERDEGEM: Yeah, we have found this almost-complete roll of British barbed wire.

PAUL REED: Well it's certainly barbs, but, you are right, it definitely looks like British barbed wire.

Barbed wire was an American invention, used for agricultural purposes, to contain cattle on the big open prairies. But, by the American Civil War, it had a military use: it could be used to defend positions on the battlefield. And that use increased throughout the course of the next century, until the First World War, when it was used on a massive scale. There was literally thousands of miles of barbed-wire fences.

A lot of people think that when they find pieces of barbed wire like this, is it modern agricultural wire, but as soon as you put your fingers on it you can see, because if it's a modern barbed-wire fence, you can probably get your whole hand in there. But here you can barely get two fingers, because they didn't want you to get some grip on it, so you can cut it.

The "devil's rope," as it became known, was used on an increasing scale, and some evil-looking barbed wire, even razor wire being used by the Germans in some parts of their line. And troops became obsessed with it, obsessed with hanging on the barbed wire.

NARRATOR: Barbed wire would stop advancing troops. There, they would face another American invention, the Maxim machine gun, which was used by both sides. The Maxim gun was the first self-powered machine gun. When a bullet is fired, it produces a recoil; a spring uses that energy to eject the spent casing and to fire the next bullet. Although the Maxim gun was heavy and required water-cooling, it could fire up to 600 rounds per minute, equivalent to 30 contemporary rifles.

Stuck on the barbed wire, soldiers would be slaughtered by the efficient killing machine.

At Bayernwald, a German front line trench has been restored in the same spot where it was first dug, on the Messines Ridge, in November, 1914.

PETER DOYLE: (Military Historian): This is a really good example of the kind of thing that they would have to build and live in, on a day-to-day basis.

NARRATOR: Military historian Professor Peter Doyle has studied the distinctive way the Germans constructed their trenches early in the war.

PETER DOYLE: The trenches, themselves, had the sides held back by wattle and brushwood. And what they would have done is bind this wood together to hold back the mud, which was continually collapsing into these trenches.

NARRATOR: Mud wasn't the Germans' only problem. The British were constantly shelling the trenches, and the real danger, in this early war trench design, was when a shell fell right inside.

PETER DOYLE: What you can see in this trench are some fatal flaws. The trench is quite wide, which is good because it gives good access, but it is also quite long, and if a shell exploded here, the blast radius would kill most of the occupants.

So, this is a pretty dangerous place to be. I can't imagine that Germans would really like sitting in this trench waiting for this to happen.

NARRATOR: Artillery was the biggest killer in World War I. Sixty percent of all casualties were caused by shellfire. For almost three years, at Messines, there were no major assaults, just the day-to-day bombardment that slowly ground the soldiers down.

MAJOR ALEX TURNER (Irish Guards): Total silence would have been very, very rare. It's very difficult for people to imagine just how assailed your senses would have been, particularly with shellfire. And a lot of nervous disorders suffered by Great War soldiers, such as shellshock, were a consequence, not so much of mental injury, but by the way that shelling affected the nervous system and, indeed, the mind.

NARRATOR: Something at the dig is becoming apparent. Each of the circles being marked by spray paint is the in-filled crater from an exploded British shell. And the numbered markers show they are getting closer together. For the archaeologists, it's a clue they're closing in on an important target for the British artillery.

The British were aiming for the German front line, and, as the team digs, they begin to unearth a complete section of a First World War fighting trench.

PETER DOYLE: It's just phenomenal.

NARRATOR: Professor Peter Doyle has come to the Messines dig to meet Simon Verdegem and see the newly uncovered trench.

PETER DOYLE: Just by excavating a trench as small as this, we get a real sense of the true nature of trench warfare. And it's pretty exciting to see that here.

NARRATOR: We know what the trench should look like, from this rare wartime photograph of the trench that connects to it.

PETER DOYLE: Shall we get out and have a look at the other side?

NARRATOR: But what the new trench reveals is that, unlike the trenches at Bayernwald, this trench had been blown to pieces.

SIMON VERDEGEM: And the explosion has pushed the side of the trench into the middle.

NARRATOR: The trench, at some stage, received a direct hit from a British high explosive shell. Amazingly, the top of the shell is still lodged among the collapsed timbers.

PETER DOYLE: Isn't that fantastic that you see the impact fuse and, actually, the result of the impact, and it's frozen in time.

SIMON VERDEGEM: Everything that is under those boards hasn't moved since the explosion.

PETER DOYLE: I've never seen anything like that.

NARRATOR: But why was only a small section of the trench destroyed? This wartime reconnaissance film shows that the trenches were constructed with a series of rectangular switchbacks, like the top of a castle wall.

It was a design adopted by both sides to protect soldiers in the trenches on either side of a direct hit.

PETER DOYLE: This is a much more secure way of digging a trench, because it…any kind of explosion can be kept in one area of the trench.

NARRATOR: In the stalemate of trench warfare at Messines, the killing continued. Among the graves in this cemetery below the German trenches is an unusual marker, that of Robert Cuthbert, a former accountant and member of the New York Yacht Club. He died when he was 47, old for a soldier. Cuthbert was one of at least five Americans buried here who had joined the Allied cause more than two years before America entered the war.

The Germans were determined to hold onto the trenches on the high ground of the Messines Ridge, but the growing intensity of the British artillery bombardment was forcing them to build stronger defenses and entrench even deeper.

It's a fact borne out by the archaeologist's latest discovery, an extraordinarily well-preserved trench, leading to a concrete structure, a German bunker.

The bunker's walls are over 30 inches thick, strong enough to shelter infantry from all but the biggest shells. If the Allies were to take the ridge, dozens of bunkers like this would have to be neutralized.

But for Simon Verdegem and the team, the most exciting discovery is the best-preserved and deepest trench ever uncovered in Flanders.

SIMON VERDEGEM: So, we are standing in the main trench leading towards the entrance of the bunker. We are actually standing on the original floorboards, which are in a remarkable good condition.

PAUL REED: It's amazing, the condition of this trench. I've never seen an original trench of this depth ever uncovered on a battlefield or ever been in one.

SIMON VERDEGEM: Over here, we have two things which are very special. On our left side, we have a board to put our rifle butts in.

PAUL REED: So, this is a rifle rack?

SIMON VERDEGEM: Yeah, exactly. It's a rifle rack, which could hold six rifles, which we think indicates that this bunker had room for six soldiers.

Incredible to see, here, these original boards and the original steps going down into this First World War dugout, with the floorboards still there—incredible.

NARRATOR: In one of the trenches, the archaeologists find evidence that the Germans were fully expecting to fight at close quarters.

SIMON VERDEGEM: It looks like hand grenades, the German hand grenades. We have one, two, three, four…at least 10 German hand grenades.

NARRATOR: Because the grenades are still live, Verdegem calls the Belgian bomb squad, who will come and make them safe. One hundred years on, these weapons are still lethal.

ALEX TURNER: Hand grenades were the weapon of choice for an infantryman in the First World War, in the attack. And here we have a German stick grenade, known as a potato masher, same nickname in German. It has an explosive charge on the end of a wooden stick which allows you more leverage in the throw and allows you to lob it more accurately.

NARRATOR: The stick grenade was introduced in 1915, and its simple design lasted into World War II. It works by placing a detonator in an explosive charge, packed within the head. The detonator is attached to a cord, which runs through the hollow base. Pulling the cord set a five-second fuse burning.

ALEX TURNER: It has an idiot's guide on the handle, which tells you how long the fuse is going to last once you have initiated it. You unscrew the base, and a little string emerges, which, when tugged, which, effectively, strikes a match in the base of the grenade head, there, and gives you your five-and-a-half seconds to lean back, take an aimed throw and lob it in the direction of attack.

NARRATOR: But there was one weapon used at Messines that, more than any other, would come to symbolize the real horror of the First World War.

SIMON VERDEGEM: It might be a gas shell.

NARRATOR: In June, 1916, the Germans released phosgene, a type of poisonous chlorine gas, from canisters sited here, above the dig. Heavier than air, it flowed down the 500 yards of no-man's land, towards the British trenches at the bottom of the Messines Ridge.

Just after midnight, British sentries observed the gas clouds in the bright moonlight. The noise of the gas rattle alerted the British troops that an attack was imminent. They would have had just minutes to put on their protective gas hoods—equipment which would bring its own set of problems.

DAVID WHITHORN: It's absolutely terrible. You can't see. It mists up almost instantly. The eye pieces go everywhere, and as soon as you breathe the wrong way, it steams up. It's already hot in here. Now you've lost all your side vision. You can't see any more than what's right in front of you. You would be turning to your right, turning to your left, seeing where the enemy is coming from.

NARRATOR: The release of gas usually preceded a German attack, so, that night, British artillery laid down a barrage, in order to prevent any German troops from reaching their lines.

David Whithorn is a military historian. What he has just done, removed his gas hood, is exactly what some of the British troops did that night, some 20 minutes after the gas release, believing the attack to be over. Unbeknownst to the British, the Germans were about to launch a second wave of poisonous gas, this time catching many soldiers unaware. By the time they got their gas hoods back on, for many, it was too late.

DAVID WHITHORN: The first thing you did was to deposit the contents of your stomach, vomiting into your gas hood. So, your gas hood would be half-full of your own vomit, and maybe the breathing tube would be blocked with your own vomit. It must have been absolute hell on earth. Wearing this, fighting for your breath, awful smell inside, fear of the gas, fear of the Germans coming. You, then, seriously consider ripping this from your head, and, when you did so, you would be then, obviously, faced with the toxic gasses in their purest form. Some soldiers fought so much to get their gas masks off that they tore their own throats apart. Phosgene is not an instant gas. It can take 24 hours to come into effect. It's slow, it's insidious, it worsens. You drown on dry land. You know you are going to die, a horrible, horrible and painful, agonizing way to die.

NARRATOR: Of the 500 casualties of the German gas attack, many are buried here, just yards from where they fell.

At the dig site, Simon Verdegem and the team have uncovered something completely unexpected. Here, just below the surface, are the remains, not of a trench, but a tunnel.

SIMON VERDEGEM: The tunnel was completely a surprise for us. We didn't know about the existence, wasn't on any map, neither on aerial photograph, so it was actually a kind of luck that we even found it.

NARRATOR: The tunnel is a sign that the Allied attempt to break the impregnable German defenses at Messines had moved into a much more deadly phase.

The team removes the roof to enter the tunnel, for the first time since it was abandoned.

SIMON VERDEGEM: The tunnel is in almost perfect condition. The walls, the roof, the floor is as it was 100 years ago. You can even see, on some points, the nails sticking out to put some kind of lights on, there. It's an amazing feeling to be able to walk through a tunnel which had been there for over 100 years and probably not been entered since 1917.

These are the roof parts from the tunnel behind me.

PAUL REED: They are extremely thick pieces of timber, incredibly good condition.

SIMON VERDEGEM: Yeah, it's amazing and it's still heavy so it didn't dry out or anything.

PAUL REED: Premade by the look of it.

SIMON VERDEGEM: All premade, so they all have the same size. They are all as long and as wide as the others.

PAUL REED: The construction system that the Germans have used here, to build these tunnels, is to dig underground. This isn't cut and cover. They have actually tunneled underneath the surface, in this case, part of the battlefield, part of the road, and constructed a tunnel there, which is then lined with these prefabricated timber sections, a section to go on the floor, two uprights, and then a roof, so it can be put in place very quickly.

NARRATOR: The sophisticated prefabricated tunnel is evidence of a widespread underground war, which reached its peak here in Messines in 1917. It was a type of warfare the Allies would develop on a massive scale.

PETER DOYLE: What they were trying to achieve by going underground was to blow up the enemy, basically to try and destroy strongpoints.

NARRATOR: If the Allies were going to break the deadlock of trench warfare, they would have to take the German strongpoints.

PETER DOYLE: We're just moving up from the British front line in front of the hills of Messines. This is the Messines Ridge, and as we rise up this slope, we can really get the sense of where the Germans were sitting, at the top of this ridge. And they would have been looking down upon the British below, and any of the enemy who wanted to attack the enemy would have to labor up this slope, just as we are laboring up it in a modern car.

NARRATOR: So, in January, 1916, the Allies forged a new plan. If they couldn't get across no-man's land, they would go beneath it. They would dig tunnels towards key targets and plant explosives below them. Then, they would simultaneously detonate the charges right beneath the Germans. It was called "earthquaking" the ridge.

PETER DOYLE: The British were in a really good situation, because they were in the lower ground, and the Germans were in the high ground. And that meant, for the British, all they had to do, in effect, was dig in a straight line. They could go in a shallow tunnel, downwards. They could undermine the Germans.

NARRATOR: The plan called for up to 49 tunnels dug under the ridge, ending in 25 mines placed under the German strongholds. The British tunnelers, called moles, included many ex-coal miners. All were volunteers, but because they were specialists working at an unusually perilous job, they were each paid three times more than an ordinary soldier.

PETER DOYLE: They were using a technique called "clay-kicking." The man who was doing the digging would lie back on a wooden construction. He would be starting at the base of a face, and, using a tool called a grafting tool, he would work out a clod of earth, clod of clay, which would then be taken away. The British used it to great effect here in Messines.

NARRATOR: The tunnelers experimented with various charges before settling on a new explosive called ammonal, a mix of aluminum powder, the fuel for the explosion, and ammonium nitrate, the chemical combustion that powers the blast. Ammonal was three times more powerful than gunpowder, and considerably less volatile. It could not be set off by fire or bullets.

Detonators were placed in some of the boxes, which were electrically fired from back in the trenches. The shockwave from the initial explosion would set off the other ammonal charges.

The drawback with ammonal was that it degraded when it was wet, so the explosive was sealed in rubberized tins and boxes before being laid in vast quantities at the end of the tunnels.

The farm of Petite Douve is just 500 yards from the dig site. During the war, Petite Douve was a stronghold on the German front line. It was also the destination for one of the British tunnels, which branched under the farm, its ends packed with explosives.

ALEX TURNER: Where I'm stood now is on the old German front line, on the western extremity of the Petite Douve strongpoint. The British front line was 500 meters west, just below those farm buildings. And that's where the British started their mine, in and among where those farm buildings are now, and they would have kicked and dug underground pretty much beneath our feet as we are now, 520 meters for the principal charge, over my left shoulder, under the strongpoint.

NARRATOR: But here at Petite Douve, in 1916, the British tunnelers weren't having it all their own way. The Germans knew of the underground plan and were digging their own tunnels in an attempt to kill the British miners.

JOHAN VANDEWALLE (Mining expert): These are all pieces of bricks from the farm.

NARRATOR: Johan Vandewalle is Belgium's leading expert on the underground war.

JOHAN VANDEWALLE: Oh, it is the place.

PETER DOYLE: What have we got here, Johan. What's this? Is this the shaft?

JOHAN VANDEWALLE: This is the spot.

PETER DOYLE: Literally, beneath here, is one of the German countermining shafts that goes straight down, and it's 20 meters down.

NARRATOR: The team is here to explore the opening to the last surviving German mineshaft from the Messines battlefield.

PETER DOYLE: This is the first time that anyone has really, since the war, done any work on this, tried to identify how deep it is, what was going on. It's like an exploration into the past, isn't it?

NARRATOR: As the British were tunneling towards Petite Douve, the Germans were sinking vertical shafts, to get beneath the British tunnels, where they would lay their own explosives, called "camouflets," in an attempt to kill or disrupt the British working above them.

PETER DOYLE: Petite Douve is an interesting aspect of the Messines battle, because here we can see the Germans seeking out the British. This is a rare survivor, really. We are not going to get this chance again.

NARRATOR: The German mineshaft could be as deep as 25 meters, but what the team doesn't know is whether it will be blocked or if the water will be clear enough to actually see anything.

PETER DOYLE: Yes. What is that?

NARRATOR: As the camera descends, from the gloom emerges the timber sides of the German mineshaft.

PETER DOYLE: We are just seeing the shaft, as we go down, and we are getting some idea of features in the side wall. It's real testimony to the men who built this that it's still here and it's still in good condition.

JOHAN VANDEWALLE: Nobody was before, here.

PETER DOYLE: So, nobody has ever seen this kind of thing. This is the first time anybody has seen this since 1917.

JOHAN VANDEWALLE: This is interesting. This is porcelain for electricity.

NARRATOR: The mineshaft would have had electrical lighting and pumps to keep the water out. They also find pipes used to take fresh air to the German miners below.

PETER DOYLE: Are we seeing the ladder, there, do you think?

NARRATOR: Attached to the wall is the steel ladder, used to descend to the bottom of the shaft. From there, tunnels would lead underneath the British miners, close by.

JOHAN VANDEWALLE: Can you imagine a ladder…with your muddy hands, going down on a steel ladder?

PETER DOYLE: You couldn't really imagine descending on that ladder down to 20 meters.

JOHAN VANDEWALLE: Stop, stop, stop. Is it possible to turn the camera?

NARRATOR: As they reach the bottom of the shaft, Johan thinks he has found something.

PETER DOYLE: So, we are on the west side, now, this is the west. Try to keep this depth but try to…gently turning it around, so that we might… clockwise…

JOHAN VANDEWALLE: This looks like, for me, the gallery to no-man's land. It is possible to come a little bit back? Stop. Because this is to no-man's land…

PETER DOYLE: So, this is the direction towards the British.

NARRATOR: This tantalizing glimpse into the darkness reveals the entry of the German's fighting gallery, an offensive trench, dug to try and attack the British miners in this underground war.

The British miners finished the main tunnel at Petite Douve in August, 1916, placing 25 tons of ammonal beneath the farm. They then set about digging a branch tunnel to lay a secondary mine nearby.

But, using sensitive listening equipment, the British tunnelers picked up the unmistakable sound of German voices only yards away.

ALEX TURNER: The British heard them. This is a blind form of warfare, digging towards one another, detecting purely by sound. So, the British heard they had been detected and blew what's called a camouflet, a disruption mine, which was only moderately successful. And that provoked a response.

NARRATOR: The Germans responded with their own explosive charge, claiming the lives of at least three tunnelers, whose bodies still remain entombed under the farm. Their names are recorded on the Ploegsteert Memorial, close by.

Of the 25 mines planned by the British under the Messines Ridge, Petite Douve was the only one lost to enemy action, a fact that still resonates today.

ALEX TURNER: The main charge at Petit Douve, all 50,000 pounds of it, was never blown. It was abandoned, so it's still lying dormant, 25 meters below ground, very close to where we stood now.

NARRATOR: The unsettling fact is that beneath this small Belgian farm is, potentially, the world's biggest unexploded bomb.

Earthquaking the Messines Ridge with mines was part of a much more ambitious plan. British High command believed that the key to unlocking the Western Front lay here, in Flanders. Breaking through the line at Messines would be the first part of an operation which they hoped would lead to the defeat of Germany.

In his preparation for the battle, the British General Sir Herbert Plumer was leaving nothing to chance.

ALEX TURNER: He pulled in 2,250 artillery pieces, which, for a seven-day artillery bombardment, consumed three-and-a-half-million shells. You had 31,000 people—that's a third of the modern British army—employed solely on road-building duties.

NARRATOR: Nine-thousand-five-hundred motor vehicles were amassed. One-hundred-seventy-three miles of railway were laid. Plumer would have more aircraft over Messines than the Germans had over the entire Western Front.

And the days of horses leading the attack were over. Tanks had been first used in the autumn of 1916, with limited success. Early versions had been slow, unreliable and unbearably hot for the crew who operated them. For the impending battle of Messines, Plumer took delivery of 72 of the latest model: the Mark IV.

ALEXANDRA CHURCHILL: This was the one that the tank drivers were waiting for. There were bits that they liked. They liked that they had moved the fuel tank away from the drivers, which, obviously, if you are under shellfire, was beneficial. They have taken those stabilizers off the back: these little wheels on the back of the tank which were supposed to steer the tank but kept getting caught in everything.

NARRATOR: Although impervious to small arms fire, they were vulnerable to artillery and were prone to catching fire. And there were other shortcomings.

ALEXANDRA CHURCHILL: They were hoping that when the Mark IV came out it would just be one driver required to drive, but, actually, it was four. And it was done through this insane system of banging on the engine casing and hand signals to try and steer the thing. It took four men to get it across a battlefield.

NARRATOR: The Mark IV was a heavily armored vehicle designed to plow through the previously impenetrable belts of barbed wire, while protecting the infantry following closely behind. While rudimentary, the tank offered a way of punching through the German defenses.

Below the town, the dig offers a grim memento of the bitter fighting to come: the helmet of a German soldier and, nearby, an ammunition box containing the belt of a British Vickers machine gun.

The Vickers gun was built on the success of the earlier Maxim gun. As well as being lighter, it featured a new "muzzle booster," a cup at the end of the barrel. This captured some of the exploding gas from each shot and pushed the sliding barrel backwards, adding to the energy of the recoil that powered the gun, making it less likely to malfunction.

But it was the big guns, heavy artillery, that began General Plumers battle to take the German bunkers and strongpoints on the Messines Ridge, in the summer of 1917.

On the evening of June 6th, Allied soldiers assembled on the low ground beneath the ridge. The mines, containing over a million pounds of explosives, were now ready under the German strongpoints. Still fearful that the Germans might discover them, the tunnelers prepared for their simultaneous detonation in the final few hours.

ALEX TURNER: The troops moved into position in darkness, quietly as possible, into taped positions in the open. They were told to lie in the open and wait. And waiting and like that is a nightmare. It's interminable. Troops just want the attack to happen. And they lay down, in the open, on their tapes, waiting to go, resting head on the forearms with the helmet, dozing, imprisoned by their thoughts.

NARRATOR: The moon came out at midnight, and it was said that in Wytschaete Wood, near Messines, nightingales could be heard singing. The tunnelers checked their watches and the leads to the explosives.

At 10 past three that morning, 19 mines under the Messines Ridge were simultaneously detonated.

ALEX TURNER: The troops would have been presented by the blast where I am standing now, which was the height of St. Paul's Cathedral, rearing right up above them, these huge tongues of flame, and incredible clods of earth the size of carts, huge lumps raining down. The commanding officers and the company commanders would have said, "Right, lads. We're off. Let's go. Get up, everybody, get up." And they would have advanced up the hill, marching into this inferno.

The Germans on this ridge, if they haven't been obliterated by the blast, they rush into position. As soon as they get into their fighting positions, it's just raining of shells coming down. They talk about the fact that they were virtually deafened, useless, "indifferent to their fate," one German officer described it.

NARRATOR: Detonating the mines on the Messines Ridge created the biggest explosion the world had ever seen. Some estimates say 10,000 German soldiers were killed in the blasts.

From the moment the mines detonated, it would take New Zealand troops just over an hour to reach the outskirts of Messines. By the end of the day, the Allies had achieved all their objectives. General Plumer was delighted with how low the casualties had been in the initial offensive, although by the end of the battle, seven days later, 24,000 Allied soldiers had been killed, wounded or were missing; the German casualty figure was as much as a third higher.

But, after nearly three years at Messines, Plumer had broken the deadlock of trench warfare.

PETER DOYLE: The Battle of Messines has got to be considered as one of the most successful battles the British Army ever fought. A combination of the mine warfare, the artillery assault and the infantry assault makes this battle unique and makes it, in my opinion, anyway, the most successful of the British army's activities on the Western Front.

NARRATOR: In Flanders, nearly a century later, the craters from the detonated mines are still clearly visible. They stand as a reminder of General Plumer's bold plan to drive the Germans from the Messines Ridge.

ALEX TURNER: It was a stellar achievement to breach such an impregnable objective, but it was unrepeatable. And its planners never envisaged that it would bounce straight into some kind of incredible breakthrough: troops flooding across the northern plain and on to Berlin.

NARRATOR: In fact, the Allies advanced barely ten miles into German territory before faltering. The three-and-a-half-million shells fired during the Messines offensive had not only destroyed enemy trenches but had churned up the ground ahead, making it all but impassable. Now, one of the grimmest campaigns of the war, Passchendaele, would claim over half a million casualties, in a sea of mud. The long-awaited breakthrough had failed to materialize.

Messines may have fallen, but it would take another year of bitter fighting before the war was over and the Armistice signed.

ALEXANDRA CHURHILL: When we look at World War I, we spend a lot of time concentrating on the people that died in the war, and we don't often think of people they left behind.

One story, which has really touched me, is the story of two friends named Robert Irvine and John Corrie, from a small town in Dumfriesshire who served together throughout the war. John was actually killed near here in the Battle of Messines.

After the battle, a New Zealand soldier was walking back across the battlefield, and he actually found a diary lying on the ground. And when he opened it up and went through it, he saw that there were a number of letters and photos and personal mementos. And he saw it belonged to John Corrie, and he thought it would be the right thing to do to if he sent it back to Lizzie, John's sweetheart.

Lizzie came to Belgium after the First World War to visit John's grave. She never married. And when she eventually died, aged nearly 100, in 1991, she was buried with a photograph of him in her hand.

NARRATOR: Although the Germans had been driven out of their trenches at Messines, it would take more than a year and the arrival of American troops joining the Allies to finally bring Germany to its knees and end the war.

The archaeological dig has unlocked a hundred-year-old time capsule of the First World War. The horses and trenches, the bombs and the bullets, and the underground tunnels, snaking beneath the killing fields of Flanders, all tell of a war of huge technological change. But, above all, the dig is a timely reminder of the human sacrifice that more than anything else has come to symbolize the horrors of the First World War.