The Western Front of World War I is infamous for trench warfare, long and grueling battles fought from dug-in positions separated by no man’s land. But a lesser-known type of battle also raged underground as both Allied and German forces dug extensive networks of secret tunnels in order to plant explosive mines beneath the enemy’s feet.

The Battle of Messines in June of 1917 witnessed what was arguably the single largest explosion of the pre-atomic age, when 19 underground mines packed with an estimated 1 million pounds of high explosives erupted beneath the German line, killing untold numbers of soldiers and shattering German morale before the real fighting even began.

The Battle of Messines was one of dozens of clashes between German and Allied forces in the region surrounding the Belgian town of Ypres starting in 1914. The Germans won an early advantage by occupying a ridgeline to the east of Ypres, a high-ground position that afforded clear views of enemy troop movements and clean shots from German artillery bunkers.

By 1917, the Allies had ambitious plans to break the costly stalemate at Ypres. The first and most critical part of that plan was to take a portion of the German-held ridgeline near the town of Messines. After that, the Allied forces hoped to push all the way to the coast of the North Sea and destroy Germany’s U-boat bases.

As early as 1915, well before there were any plans for the Battle of Messines, tunneling operations were underway beneath the Messines Ridge. The Allied forces, composed of British, Canadian, Australian and New Zealand divisions, included “tunneling companies” manned with soldiers recruited for their excavating skills.

“Most of the tunnelers were coal miners or gold miners very experienced at digging,” says Ian McGibbon, a former general editor with the New Zealand Ministry of Culture and Heritage and co-editor of New Zealand’s Great War: New Zealand, the Allies and the First World War. “The idea was to get under your opponent’s lines and explode mines. It was a very nerve-wracking experience, especially when you approached their lines and knew that the enemy might explode a mine near your shaft to destroy it.”

At Messines, the Allies first dug shafts closer to the surface to divert attention from the deeper shafts that actually held the mines. German tunnelers took the bait and detonated charges to collapse the decoy tunnels, wrongfully thinking they had defused the threat from below.

In reality, companies of British, Canadian and Australian tunnelers had successfully dug and armed 22 separate mine shafts beneath the Messines Ridge, each packed with tens of thousands of pounds of ammonal, a highly explosive combination of ammonium nitrate and aluminum powder.

The Battle of Messines began at 3:10 a.m. on June 7, 1917, when 19 of the 22 Allied mines detonated beneath the Messines Ridge. It’s impossible to know how many German soldiers were killed instantly in the mushroom cloud of earth and fire that erupted from 80 feet below. The commonly cited figure is 10,000 deaths from the explosion alone, but McGibbon believes that number represents the total number of German fatalities for the entire three-day battle.

A British artillery officer, Ralph Hamilton, witnessed the explosion and compared it to the famously horrific battle at the Somme.

“First, there was a double shock that shook the earth here 15,000 yards away like a gigantic earthquake,” wrote Hamilton. “I was nearly flung off my feet. Then an immense wall of fire seemed to go halfway up to heaven. The whole country was lit with a red light like in a photographic dark room. At the same moment, all the guns spoke and the battle began on this part of the line. The noise surpasses even the Somme; it is terrific, magnificent, overwhelming.”

Early newspaper reports from London claimed that the prime minister himself was awakened by the thunderous rumble of the mines exploding at Messines, 140 miles from 10 Downing Street. The truth, more likely, was that the prime minister and others across the English Channel heard the roar of the Allied heavy artillery, which followed the earth-shattering explosion with a synchronized barrage of 2,000 guns.

The mastermind behind the Battle of Messines was Field Marshal Herbert Plumer, the British officer in charge of the Second Army at Ypres. McGibbon calls Plumer “meticulous” for the way that he carefully planned, trained and executed the artillery attack at Messines.

By 1917, Allied engineers had greatly improved the science of sound ranging, which employed a network of battlefield microphones to calculate the precise location of enemy artillery guns. Armed with this technology, Plumer spent the weeks before the attack precision-bombing German bunkers and pillboxes to clear the way for a charge across no man’s land.

But Plumer showed his real genius right after the massive explosion that launched the attack. The roar of 2,000 heavy artillery guns heard in London was the opening salvo of what’s known as a “creeping barrage.” The gutsy technique, developed at the Somme but perfected at Messines, was to rain down artillery directly in front of a line of charging infantrymen.

“As the troops are advancing, there’s a curtain of fire coming down 50 yards in front of them,” says McGibbon. “It must have been a fearsome experience for the Germans, seeing this coming toward them.”

For the already shaken German soldiers who survived the apocalyptic explosion, the thunderous charge of the creeping barrage was too much to bear. The Allies crossed no man’s land with few casualties and easily captured Messines Ridge. Over the next two days, the Germans staged a fierce counterattack that claimed thousands of Allied casualties, particularly among the New Zealand division stationed at Messines, but they bravely held on to the ridge.

The Battle of Messines is widely considered one of the greatest Allied victories of World War I.