The failure of the Nivelle Offensive and the widespread mutinies in the French army forced the hand of the British Army. Sir Douglas Haig ordered his commanders to launch attacks along its front to draw away the German reserves, lest they exploit the French situation. The first operation was to reduce a small German salient on Messines Ridge opposite the British Second Army. The British and Canadians of the Second Army had been preparing for an offensive since the trenches stabilized after the Second Battle of Ypres nearly two years before. Recently reinforced by the Australians and New Zealanders, the Second Army had more than twenty mines painstakingly dug underneath Messines Ridge. Because of the geological conditions of the area, the mines took nearly a year to complete.
Due to the high water table of Flanders, each mine had to be dug straight down about 150 feet through the waterlogged “sandy clay”, to the “Bastard” clay mix, or “Paniselien Clay” which was impermeable to water. Then the mines had to be dug horizontally underneath No Man’s Land to the German lines. The issue was the great force the sandy clay placed on mine shafts. The tunnel companies of the Second Army lost more than a few men before they figured out that the vertical shafts had to be lined with specially made steel walls to prevent “flooding”. If one ruptured, then it would fill the entirety of the shaft and mine with thick oozing sandy clay, killing everyone inside, and requiring the tunnelers to start over in a different spot.

Throughout 1916 and 1917, the British mined underneath the Messines Ridge, and the Germans countermined, though the Germans did not realize how the extensive the British mining operation was. As the British dug horizontally, the Germans in several places dug vertically. Where they intersected involved vicious hand to hand combat with shovels, knives, and pick axes that would not have been out of place in the Middle Ages and the Renaissance. The biggest danger was flooding by the Germans, inadvertently at first but deliberate later on: the Germans used timber for their shafts through the sandy clay, which ruptured regularly. If a German shaft connected with a British mine, the timber virtually guaranteed the entire British effort would be eventually flooded with the sandy clay “ooze”. By June 1917, the British had 25 mines underneath the Messines Ridge which the Germans were unaware of. They packed them with nearly 600 tons of explosives.

The British planned to detonate them on 7 June 1917 to seize the ridge as part of the reduction of the Messines Salient. This assault was a prelude to a general offensive in July. The night before, the chief of staff of the Second Army told the press, “Gentlemen, we may not make history tomorrow, but we shall certainly change the geography”.

Due to the heavy preparatory bombardment, the Germans were expecting the attack. However, they weren’t prepared for what came next. At 3:10 am on 7 June 1917, 19 of the mines were detonated. The other six were packed with explosives, but “lost” in the tunnel warfare, couldn’t be detonated, or were underneath positions that were evacuated (These would pose significant problems after the war, and several still haven’t been found). The resulting explosion was heard by the British Prime Minister as he toiled away at his desk in London, and as far away as Dublin, Ireland. It was the loudest recorded event in history until the atomic bomb exploded over Hiroshima 28 years later. The explosions knocked over just about every soldier and officer in the Second Army. They opened great craters in the Messines Ridge, and instantly killed nearly ten thousand German troops.

The remaining stunned Germans were in no shape to resist the subsequent assault. The British, Canadians and Australians all incorporated the lessons learned from the Canadian assault on Vimy Ridge. The Allied troops at the Battle of Messines seized all of their initial and subsequent objectives within hours. The Germans counterattacked but were unsuccessful.

The Messines Salient was in Allied hands, but there was no plan to exploit the success. Nonetheless, the Battle of Messines was the first Allied victory in the Great War where defensive casualties outnumbered offensive casualties and was a much needed boost to Allied morale.

Of the remaining mines, a thunderstorm detonated one in 1955, luckily killing just a cow. The remainder are still being located.