Deep beneath the blood-soaked fields of Flanders and the Somme, a silent and savage war raged, one that made the horrors of the trenches above seem almost tolerable. For two years, from 1915 to 1917, thousands of soldiers on both sides of World War I fought and died in darkness, buried alive in narrow, dripping tunnels lit only by candlelight, digging toward an enemy they could not see but could hear. This was tunnel warfare, arguably the worst job in a conflict already defined by unspeakable suffering, and its legacy lingers today, with more than 80 tons of live explosives still buried under the Belgian countryside, waiting.
The war on the Western Front had ground to a halt by late 1914, with a 400-mile line of trenches stretching from the Swiss border to the North Sea. Neither side could outflank the other, and frontal assaults across open ground were shredded by machine guns, artillery, and barbed wire. Commanders sent waves of men to die, but the deadlock remained unbroken.
Then came a desperate idea: go under the killing fields, dig beneath the enemy, and blow them up from below.
The Germans struck first, using trained engineer troops to dig shallow tunnels and detonate small mines under Indian Corps trenches in December 1914. The blasts ripped along nearly a thousand yards of trench, killing over 800 men in minutes. The British had no answer.
German mining became a relentless threat, and the high command could no longer ignore it.
Enter John Norton Griffiths, a self-made civil engineering contractor whose firms built railways, harbors, and tunnels. In late 1914, his men were digging sewer tunnels under Manchester using a specialized technique perfect for soft clay. When he heard about the German mines and the British struggle in Flanders’ wet soil, he offered to bring his sewer workers to France.
He was ignored at first.
The problem grew severe enough that the British finally listened. Griffiths introduced a revolutionary method: clay kicking. A digger, called a kicker, lay on his back on a wooden cross-shaped frame, reclining at about 45 degrees with feet toward the face.
He used a sharpened spade-like tool with footrests, driving it into the clay with his legs, twisting out a lump, and passing it back over his head.
Behind the kicker, a bagger filled sandbags with the excavated clay, while a trammer wheeled the bags to the surface on a small trolley, returning with timber for the walls. The timber supports were cut without nails or screws, assembled so that moisture would make the wood swell and lock into place. No hammering meant no noise to betray the operation to enemy ears.
Clay kicking was silent and fast, achieving rates of about eight meters per day compared to the Germans’ two meters. But it required firm, plastic clay like the blue clay layer under Flanders’ wet surface. Further south, in the Somme and Arras regions, the ground was chalk, which drained well and needed no timber but had to be dug with picks, a slow, noisy process that the enemy could hear.
The Germans never developed clay kicking, relying on mattocks for the entire war. Once British tunneling companies were established, they seized the advantage, turning the underground war in their favor. The basic tactic was to dig a shaft behind your own front line, drive a horizontal gallery under no man’s land, reach the enemy trench, pack a chamber with explosives, tamp the tunnel with sandbags to direct the blast upward, and fire it electrically at the chosen moment.
The explosion destroyed the trench and left a crater, which infantry rushed to capture before the enemy recovered. Crater lips often provided elevated views of enemy lines, sparking savage battles for possession. By 1916, large mines were regularly blown in the opening minutes of major offensives.
Tunneling companies were formed at unprecedented speed. Men who had been civilian sewer diggers found themselves under no man’s land in as little as four days. Skilled miners were paid six shillings a day, four times an infantry private’s pay, causing arguments but reflecting the extreme danger of the work.
Canadian, New Zealander, and Australian tunneling companies were soon formed. The underground war spiraled out of control. In 1916 alone, about 1,500 mines were exploded on the British front, plus thousands of smaller underground charges that never broke the surface.
At the peak, 227 mines were blown in a single month, one explosion every three hours.
All of this happened in tunnels about four feet high and under three feet wide, lit by candles, with men working six-to-eight-hour shifts. The only way to find the enemy was by sound. Both sides fought a war against noise.
British miners wore soft-soled shoes instead of hobnailed boots. Their trolleys ran on screw-mounted rails, and by 1916, every trolley used self-oiling bearings to avoid squeaking.
Each tunneling company kept specially selected listeners who worked at the end of short side tunnels, called listening posts. In early days, they used the naked ear, watching water in a glass for vibrations, or biting on a stick to feel tremors. From 1915, the British had the geophone, two wooden discs connected by rubber tubes to stethoscope earpieces.
Placed on the tunnel floor, a listener could take a compass bearing on enemy digging up to 50 meters away.
These men lay or knelt alone for hours in cramped, black, dripping tunnels, straining to hear picks and shovels. When enemy digging suddenly stopped, it often meant the enemy had finished his tunnel and was now loading explosives. The cat-and-mouse game of counter-mining began.
The camouflet became the most feared underground weapon: a small charge designed to collapse a pocket of ground without breaking the surface, burying or gassing enemy miners. Attack tunnels rarely reached the enemy line; they were found and collapsed by counter mines somewhere under no man’s land.
Methods grew darker. After blowing a first charge into an enemy gallery, a second charge could be timed to 𝓀𝒾𝓁𝓁 the rescue party digging out survivors. Both sides occasionally detonated very large charges to wreck the ground itself, making further tunneling impossible.
The most terrifying scenario was when opposing tunnels accidentally met, leading to hand-to-hand combat in darkness and cramped space. Tunnelers carried knives, pistols, and shortened rifles with cut barrels and stocks for close encounters.
But the biggest killer was invisible: carbon monoxide. Every explosion from a mine, camouflet, or surface shelling produced the colorless, odorless gas. With tiny air spaces and no proper ventilation, men collapsed without warning.
Survivors were left with permanent mental damage. Gassed half-conscious men were known to fight the rescuers trying to drag them out.
Mice were used as early detectors, but canaries became standard. Their fast metabolism and high oxygen demand made them collapse from carbon monoxide before a man would, signaling it was time to evacuate. Birds in cages were carried down shafts and watched diligently.
The British set up mine rescue schools. Rescuers, called protomen after the Proto breathing set, wore self-contained oxygen rebreathers giving up to two hours of air. They were hand-picked experienced miners chosen for calm under pressure.
When a gallery was blown in or gassed, they went down with lifelines and stretchers, bringing out survivors through ground that could collapse again at any moment.
Even without enemy action, tunnels collapsed from unstable ground, shellfire, or shockwaves from nearby blasts. Galleries flooded, especially in Flanders’ wet ground, where a breached water layer could drown a tunnel in minutes. Whole shifts were lost at once.
William Hackett, a 43-year-old coal miner rejected by the infantry three times for his age, was taken by the tunnelers. In June 1916 near Givenchy, a German mine brought down his gallery, trapping him and four others 40 feet underground. A rescue party dug for two days and opened a small hole.
Hackett pushed three men through but refused to leave Thomas Collins, a badly injured 22-year-old Welsh soldier who could not move.
The hole shrank, and the gallery collapsed completely. Rescuers dug for four more days under German shelling, but the two men were never reached. Hackett received a posthumous Victoria Cross, the only one ever awarded to a tunneller.
He and Collins lie under the fields of Givenchy to this day.
The worst single example of men sealed underground happened in the Winterberg Tunnel, a German communication and shelter tunnel over 300 meters long and 20 meters deep, built so troops and supplies could move without being shelled. In May 1917, French observers in a balloon pinpointed both ends. Heavy artillery set off ammunition stockpiled inside and collapsed the southern exit.
The tunnel was sealed with some 270 men inside. No rescue was possible because the area remained under French attack. The trapped men died over about six days.
Only three got out alive through a small breach. The rest remain in the tunnel to this day.
Ammonal was the standard British explosive for large mines. A mixture of ammonium nitrate, aluminum powder, TNT, and charcoal, it created a supersonic shockwave with vastly more destructive power than gunpowder. It was cheap and safe to handle, but its big weakness was moisture.
Wet ammonal would not detonate, so charges were sealed in soldered metal tins or rubberized bags.
While the underground war on the Western Front grew, similar battles raged elsewhere under even worse conditions. At Gallipoli in 1915, opposing trenches were only a few yards apart. Both sides mined constantly at close range, men regularly blowing straight into each other’s galleries.
The most extreme version was fought in the Alps and Dolomites on the Italian-Austrian front. That mountain front, much of it above 2,000 meters, was deadlocked. Both sides dug into the mountains, carving galleries, barracks, and gun positions into solid rock.
On the Marmolada glacier, Austrians cut a tunnel city of several kilometers inside the ice.
Unlike Flanders clay or Somme chalk, the Dolomite rock required hand drills and chisels. Whole mountain tops were blown off. The largest single mine of the entire war is credited to this front: an Austrian charge of 50 tons of blasting gelatine fired over the Italians on Monte Pasubio in March 1918.
Back on the Western Front, the mines set for the first day of the Battle of the Somme, July 1, 1916, showed how far the method had come. British tunneling companies laid 19 mines under the German front line, eight large and 11 small. The biggest were at Hawthorn Ridge with about 40,000 pounds of ammonal, and at La Boisselle with the Y Sap mine holding another 40,000 pounds and the Lochnagar mine holding two linked charges of 36,000 and 24,000 pounds.
The Hawthorn Ridge mine became famous because it was filmed from about half a mile away. The craters are still there today, with human remains in the ground around them.
To understand the peak of this underground war, you need to understand the ground itself. Under much of Flanders, the ground came in three layers: topsoil of wet sandy clay, then a treacherous waterlogged layer of running quicksand, and beneath that, dry firm blue clay ideal for deep tunneling. The Germans concluded that deep mining through the sand layer was impossible.
They held onto that belief until 1916.
The British borrowed a technique from civilian mine engineering called tubbing. Instead of timber, the shaft was lined with prefabricated cylindrical sections of steel bolted together, forming a continuous watertight metal tube through the wet layer. Once the tube reached the dry blue clay, miners switched back to clay kicking.
The Germans, convinced the sands could not be crossed, did not believe deep British galleries could exist under their positions. They concentrated their countermining at shallower levels, where the British kept them busy with minor feint attacks.
The peak of the underground war came a year after the Somme, at Messines Ridge in Belgium, south of Ypres. The ridge was German-held high ground overlooking the British lines. The idea of destroying it from below started in 1915.
British, Canadian, Australian, and New Zealand tunneling companies dug under it for roughly two years, driving deep galleries through the blue clay, some over 600 meters long and up to 38 meters down.
In total, more than 20 mines were prepared and charged with a combined load of around 450 tons of ammonal and gun cotton. Germans found and destroyed one British mine under La Petite Douve Farm with a camouflet in August 1916, and their tunnelers came within meters of several other charged chambers without ever realizing what sat next to them.
The German command misjudged the danger. In the weeks before the attack, they were convinced the ridge was safe. At 3:10 a.
m. on June 7, 1917, after 20 minutes of eerie silence, nervous German troops fired flares to see what was happening. Then 19 mines went up along a roughly six-mile stretch of the ridge over the space of about 20 seconds.
The combined explosion ranks among the largest man-made explosions before the atomic bomb. The blast was heard as far away as London. British infantry took the entire ridge within hours, along with over 7,300 German prisoners, many dug out dazed from the wrecked earth.
After Messines, the underground war wound down fast. The front was open for the first time, and fighting became more mobile toward the end of the war. However, the war ended with live mines still in the ground.
Not every charge prepared for Messines had been fired that morning.
The mine under La Petite Douve Farm, holding 50,000 pounds of explosive 80 feet down, was abandoned after the German camouflet attack wrecked its gallery. One of the galleries under Peckham Farm, loaded with about 20,000 pounds, was abandoned when it collapsed after pumps failed. A group of four more mines, dug under a German strong point called the Birdcage near Ploegsteert Wood, was deliberately left unfired because the Germans had pulled back just before the attack.
When the German spring offensive of 1918 overran the area, British headquarters records relating to the mines were lost. Altogether, six large abandoned charges, totaling over 80 tons of explosive, lay forgotten with nobody knowing where exactly they were.
Then, on July 17, 1955, 38 years after it was laid, one of the four Birdcage mines, about 32,000 pounds of ammonal, went off on its own near Le Pelerin in Belgium. Electricity had come to the area after World War II. A pylon was erected right over the forgotten mine.
Lightning struck it, and the current apparently ran down and reached the detonators. The blast tore out a large crater, wrecked the pylon and a stretch of road. The only casualty was a single cow.
Experts were surprised the ammonal still worked. The careful wartime waterproofing, with explosives packed in tins under tarred canvas and detonators sealed in bottles, had kept the charge live for four decades. The other five charges are still down there, around 150,000 pounds of explosive still underground in unknown condition.
No one knows whether or when it could be activated.
