This article is a follow-up to a piece on World War I's Battle of the Somme, which includes more background on the formation of the tunnelling companies within the British Army

Offensive mining by the British Army against the German-held Ypres Salient near the village of Messines in West Flanders, Belgium, had begun in spring 1915, with the tunnels being dug approximately 4.6-6.1m deep.

While the chalk downs of the Somme were relatively easy to tunnel through, as the chalk drained quite well and shoring did not have to be used along the entire length of the tunnels, the underlying geology in Flanders was different. The surface layer there was of water-bearing clay, so any tunnelling through it was difficult, dangerous and slow; pumps had to be used to remove the water, and the tunnels had to be shored up to prevent them from collapsing.

However, underneath this water-bearing layer in Flanders was a layer of blue clay. The blue clay still needed to be shored up, but it did not hold water, so much of the tunnelling was done through this layer.

Clay-kicking

The shallower tunnels were necessary, as they were dotted with listening posts for the detection of German countermining; however what the German Army did not know was that there was a deeper system of tunnels being driven into the clay, far below the shallower tunnels that they had identified. In addition, the British deliberately diverted the attention of German miners from their deepest tunnels by making many minor attacks in the upper levels.

The concept of a deeper mining offensive was devised in September 1915 by Brigadier George Fowke, the engineer-in-chief of the British Expeditionary Force, who proposed to drive galleries 18-27m underground. Fowke had been inspired by Major John Norton-Griffiths, a civil engineer, who had helped to form the first tunnelling companies and introduced the quiet and specialised tunnelling method called clay-kicking as a military mining technique.

Clay-kicking - also known as ‘working on the cross' - had been developed in the UK for driving tunnels through clay-based geology for sewer, road and railway works (including significant parts of the London Underground system). It used leg power to dig rather than hand tools such as picks or mattocks, which meant that less effort was expended, and work in the deeper tunnels could progress much more quickly.

Norton-Griffiths was an engineer who had carried out major contracts in Africa and South America, as well as in the UK. He wrote to the British War Office in early December 1914 to suggest that his tunnelling workers, who were working on a contract to refurbish the main sewer in Manchester, could be useful for the war effort. His letter was not acted upon at the time; however, following a German mine attack under British lines on December 20 that year which killed an entire company of 800 men, the British Army decided to create its own tunnelling companies as a response.

Lord Kitchener, Britain's Secretary of State for War contacted Norton-Griffiths in February 1915 to take him up on his suggestion; the first group of clay-kickers, which Norton-Griffiths referred to as his ‘Manchester Moles', were sent to France by the end of month as some of the founding members of the 170th Tunnelling Company of the Royal Engineers. They left Manchester on a Thursday, and by the following Monday they were already working underground at Givenchy.

The other miners were quick to pick up the clay-kicking technique. The British military historian Lyn MacDonald collected first-hand accounts in her book ‘They Called it Passchendaele - the story of the third battle of Ypres and of the men who fought in it'; one of these was Corporal Tom Newell from the 171st Tunnelling Company, who said: "To be a good clay-kicker you had to be long-legged, young and strong. At the age of 21 I was all three. You lay on a wooden cross made out of a plank with the cross-strut just behind your shoulders. The cross was wedged into the tunnel so that you were lying at an angle of 45° with your feet towards the face. You worked with a sharp-pointed spade with a foot-rest on either side above the blade, and you drove the blade into the clay, kicked the clay out, and on to another section, moving forward all the time. With the old broad-bladed pick we could only get forward at best 6ft [1.8m] on every shift, but when the clay-kicking method was introduced we were advancing as much as 12ft or even 14ft [3.7-4.3m] on a shift."

The clay-kicking teams consisted of several people - the ‘kicker', who prised out the clay at the face using the power of his legs; the ‘bagger', who filled sandbags with the excavated clay; and a ‘trammer', who filled a small trolley on rails that would take the spoil back to the shaft. The filled bags were hoisted up the shaft and deposited on the surface to give extra protection to the men in the trenches, and the trolleys would bring timber into the tunnel on their return journeys. To encourage drainage, the tunnels were built on a slight uphill gradient of between 1:100 and 1:50.

For every 9in (22.9cm) excavated, a timber ‘sett' was installed to shore up the tunnel. Due to the need for silent working, nails and screws were not used to fix the timber in place - they were sawn with small rebated steps so that the geophysical pressure of the surrounding clay would hold them in place. However, the length of time that the tunnels were kept open led to the swelling of the clay and the crushing of support timbers in some of the tunnels, which resulted in problems for the engineers.

The virtual silence of the clay-kicking method was one of its great advantages. Digging with hand tools required striking the clay, which made noise that could be detected by German listening posts; however the German Army was not aware of clay-kicking and it continued to use hand digging for its own tunnelling.

Secret tunnelling

Early in 1916, the digging of deeper tunnels began in the Flanders region. The miners sunk shafts wherever they could find cover, including in the yard of a destroyed farm and inside the bricks left at the site of a derelict cottage; in many cases the tunnels descended at an angle from a sandbagged emplacement inside a reinforced trench behind the front line. First the shafts were dug through the heavier brown clay, deeper through a stratum of water and sand, and deeper into the blue clay where the tunnels were planned to be deeper and longer than ever before.

The tunnelling companies took great care to prevent the German Army from knowing that they were tunnelling in the deeper layer of blue clay, as if it were spotted it would remove the element of surprise. Captain M Greener of the 175th Tunnelling Company recounted to MacDonald: "You'd shift 30 or 40 sandbags in a shift, or more, and you couldn't dispose of the dirt during the daytime.  It had to come out at night. You had to be very careful about the disposal of the dirt if you were mining in blue clay, because the stratum of blue clay was deep down, far below the level of the ordinary trenches. If you collected a pile of sandbags with the blue clay you had to get rid of it quickly, because the Germans would shell it just to see what was in it.

"You had to take it out and distribute it as best you could at night, behind the trenches, well away from the line, so that the trenches didn't look any different. That was the great thing. And that was the big problem, the disposal of the dirt from the mine. But we had carrying parties from the infantry… It was no wonder they called them the PBI [Poor Bloody Infantry]!"

Lance Corporal John Wilson wasn't a miner, but was in the Durham Light Infantry, which held the high ground known as Hill 60 for a long time in 1916; a mine was being excavated underneath it. He told MacDonald: "We'd be put on these working parties to help the REs [Royal Engineers]. We carried the sandbags in empty and we carried them out full. And we had to carry them some distance! You had to get them well away, far back from the line. Sweating like pigs, working all night in among the shelling and then back to the tunnel before daylight or, if you were unlucky, into the trenches, with the bullets flying all around you."

Tunnel boring

In an effort to speed up the progress of the tunnelling, an idea was proposed to use tunnel boring machines especially designed for this purpose. Six of these machines were recommended; an experimental prototype, based on one that was used for the London Underground, was ordered. It was built by the Stanley Heading Machine Company of Nuneaton, UK, at a cost of approximately £6,000 (which is the equivalent of over £470,000, or US$625,000, in 2016).

Weighing 7.6t, the machine featured a cutting head that was specifically designed for the hard strata of clay in Flanders. It required 24 crates to transport it to the port at Folkestone in southeast England, and three General Service trucks to transport it from the port at Boulogne-sur-Mer in France to Flanders. It arrived at its destination on February 17, 1917, but was not switched on until March 4.

The chosen site for the tunnel boring machine was the tunnel at Petit Bois, but it proved to be quite difficult to get it into the tunnel undetected as it was so close to the German line. There were light railways intersecting the front but they were designed for small wagons containing supplies and ammunition, and the 7.6t tunnel boring machine was far beyond its capacity.

MacDonald related: "Again and again, night after night, the wagons were derailed by the weight of the top-heavy crates, and the cursing, sweating engineers were faced with righting them and manhandling the huge crates back on board. It was hardly miraculous that the Germans began to get wind of a ‘secret weapon'. They did not know exactly what it was but they did know that something was up, and the guns on the ridges above gave Petit Bois their undivided attention."

Somehow the machine survived the bombardment, and under the cover of darkness it was unboxed, assembled and edged down the sloping shaft into the tunnel. When it was finally switched on, it was found to work much more quickly than the men could; it also carved a smooth tunnel approximately 6ft (1.8m) in diameter, and all the tunnellers needed to do was follow the machine and shore the tunnel up with timber, then remove the clay.

At the end of the shift, the miners switched the machine off - seven hours after it had begun its tunnelling. It would not start again, as it was caught in the mud; it took a whole day of digging to set it free.

It turned out that the machine had to be switched off at intervals to prevent it from overheating, but this presented a problem as there was no retroactive mechanism to pull it back out of the earth when it stopped. Also, the weight of the clay pressing in upon the drill was greater than the momentum of the engine designed to drive it forward. The machine was repeatedly dug out, and it repeatedly jammed. In addition to this, there were problems with its supply of electricity from small generators, which could not produce enough consistent power for the machine.

MacDonald wrote: "The fuses blew so often that the weary men who were nursing the machine along its checkered way ran out of heavy fuse-wire. When one of them, rather more weary and exasperated than his companions, also ran out of patience and mended one of the big fuses with the only wire he had to hand - which happened to be heavy barbed wire - the secret weapon gave up the ghost altogether. It had excavated just 200ft [61m] of tunnel. The experts who had designed the machine and accompanied it so enthusiastically to Flanders set off back to London with their tails between their legs, abandoning the tunnel and leaving the wonder-weapon to sit there to eternity held fast beneath 80ft [24m] of Flanders clay."

Lead up

Big chambers were excavated at the end of the tunnels, deep under the German positions, and they were tightly packed with ammonal, an explosive made up of ammonium nitrate and aluminium powder. The wires of the firing mechanisms were led back along the tunnels, and the tunnels themselves were packed back for several metres with sandbags to ensure that the full force of the explosions would fly upwards. Every day the completed tunnels were tested with electrical charges to ensure that the mines were still able to function, as there was a possibility of explosives being affected by the damp.

The British Army had been tunnelling in the area for two years, and the fact that the German Army did not know about the deep mines has been considered to be one of the best-kept secrets of WWI.

There were 21 mines planted under German lines, with a total of over 1 million pounds (453.6t) of explosives between them. The largest mine, Spanbroekmolen, was 29m underground and contained 41t of ammonal.

Zero hour, the time when the mines would be detonated, was set as 3:10am on June 7, 1917. In the week leading up to the attack at Messines, British guns and howitzers bombarded the German trenches, cut wire, destroyed strongpoints and conducted counter-battery fire against the German artillery.

The tactical objective of this attack was to capture the German defences on the Messines Ridge, and to deprive the German Army of the high ground on southern flank of the Ypres Salient. If captured, the Messines Ridge would give the British Army control of this strategically important ground, shorten the front, remove the German Army's ability to observe British positions further north, and also allow the British Army observation of the southern slope of Menin Ridge at the west end of the Gheluvelt plateau in preparation for a larger offensive.  It was also designed to relieve pressure on the French Army, in the same way that the Battle of the Somme had done the previous year.

The evening before the attack, General Sir Charles Harington, chief of staff of the British Second Army, remarked to the British press: "Gentlemen, we may not make history tomorrow, but we shall certainly change the geography".

The battle

At 2am, just over an hour before zero hour, British aircraft cruised over the German lines to camouflage the sound of tanks as they drove to their starting points. There were 80,000 British, Australian, and New Zealand infantry getting into position for the offensive; once the mines were detonated they were to advance, supported by heavy artillery. If the mines did not go off, however, the infantry were not supposed to wait - they were to go ahead as soon as the barrage of artillery fire began, and hope that the explosions were not merely delayed.

The artillery fire was lifted half an hour before dawn, and as they waited in the silence for the offensive to begin, some of the troops reportedly heard a nightingale singing.

At 3:10am, the plungers were pushed to detonate the mines. Lieutenant J Todd of 11th battalion, Prince of Wales' Own Yorkshire Regiment, told MacDonald: "It was an appalling moment. We all had the feeling, ‘It's not going!' And then a most remarkable thing happened. The ground on which I was lying started to go up and down just like an earthquake. It lasted for seconds and then, suddenly in front of us, the Hill 60 mine went up."

Rifleman T Cantlon, 21st Battalion, King's Royal Allies, also described it to MacDonald: "We could hardly believe it. We couldn't take our eyes off it. We'd only known about it a minute or so before and we could hardly believe our eyes. None of us had seen anything like it ever. It was just one mass of flames. The whole world seemed to go up in the air."

2nd Lieutenant JW Naylor, Royal Field Artillery, recounted: "Our plunger was in a dug-out, and the colonel and I were actually standing outside the dug-out because we both knew what was going to happen and we wanted to see as much as we could. The earth seemed to tear apart, and there was this enormous explosion right in front of us. It was an extraordinary sight. The whole ground went up and came back down again. It was like a huge mushroom. I can see it now! It was tremendous. One almost felt ‘Good old England'. You wanted to wave a little Union Jack. Thank God we've done something. It had a tremendous moral effect. To sit there day after day in these ghastly trenches with nothing of any importance happening and suddenly you get a major thing like that."

Philip Gibbs, who served as one of five official British reporters during WWI, also witnessed the explosions. He wrote: "Suddenly at dawn, as a signal for all of our guns to open fire, there rose out of the dark ridge of Messines and 'Whitesheet' and that ill-famed Hill 60, enormous volumes of scarlet flame … throwing up high towers of earth and smoke all lighted by the flame, spilling over into fountains of fierce colour, so that many of our soldiers waiting for the assault were thrown to the ground. The German troops were stunned, dazed and horror-stricken if they were not killed outright. Many of them lay dead in the great craters opened by the mines."

Of the 21 mines that had been set, 19 of them went off; the series of mines exploded over a period of 19 seconds, moving along the front and mimicking the effect of an earthquake. The fact that the detonations were not simultaneous enhanced their effect on the German troops. Strange acoustic effects also added to the panic - German troops on Hill 60 thought that the Kruisstraat and Spanbroekmolen mines were under Messines village, which was located well behind their front line, while some British troops thought that they were German counter-mines going off under the British support trenches.

The combined explosion was even larger than those from the first day of the Battle of the Somme in July 1916; as with the Somme explosions, the joint explosion of the mines ranks as one of the largest ever artificial non-nuclear explosions, and the sound of the explosion was reportedly heard in both London and Dublin. In Lille, 24km away, German troops were said to have run around ‘panic-stricken'.

The combined explosion also holds the dubious merit of killing more people than any other non-nuclear man-made explosion in history; it killed approximately 10,000 German soldiers between Ypres and Ploegsteert.

Aftermath

The battle itself is generally agreed to have been a tactical and operational success for the British Army. Within three hours of the mines being detonated, the Messines Ridge had been captured, and by mid-afternoon every objective had been achieved. The battle lasted a week, until June 14, but all counter-attacks by the German Army were failures. The loss of the Messines Ridge may have had a worse effect on German morale than the number of casualties. The German Army's losses over the week of the battle were 25,000 casualties, 7,500 prisoners and 67 guns, while the British Army lost 17,000 men (more than half of them from Australia and New Zealand).

In his book ‘Mud, Blood and Poppycock', author Gordon Corrigan noted: "The attack itself was a great success, but given that it was preceded by a 17+-day artillery bombardment, in which two and a half million shells were fired during the last seven days alone; that it was supported by 72 Mark IV tanks; that the British had complete control of the skies thus preventing any German reconnaissance; and that the British were attacking with nine divisions against the Germans' six, of which only two were on the ridge itself, it would have been a success anyway. Nevertheless the mines did cause confusion amongst the Germans - in Lille the tremors were thought to be an earthquake - and they certainly cheered the British attackers."

The Battle of Messines was a prelude to the much larger campaign of the Third Battle of Ypres, also known as the Battle of Passchendaele, which began in July 1917. The Battle of Messines was also the peak of mine warfare in WWI; the last British deep mine of the war was detonated by the Royal Engineers near Givenchy-en-Gohelle, France, on August 10, 1917 - following this, the Royal Engineers' tunnelling companies concentrated on constructing deep dugouts to accommodate troops.

By the time World War II started in 1939, mining warfare was not of much use as the tunnelling was too slow to keep up with the movements of troops. There have been some more recent examples of tunnelling as a military tactic, most notably by the Viet Cong during the Vietnam War, but there has not been anything approaching the scale of the military mining used during WWI.