About 8 kilometers south of Ypres, in the middle of a farm, is a small green pond known as the “Pool of Peace”, but its creation was a rather violent event.

It was 1916 and the First World War was in its second year. The Germans had occupied the Belgian coast and was using the coastal ports as bases from which they attacked merchant ships and troop transports in the North Sea and English Channel. Capturing these ports became a major objective for the British army. But before that could happen, the British had to drive the Germans out of a tactically important high ground called the Messines–Wytschaete Ridge, located south of Ypress, in Belgium.

The crater at Spanbroekmolen, also known as the Lone Tree Crater or the Pool of Peace. Photo credit: Eric Huybrechts/Flickr

In the preceding few months, the British were attacking German positions with underground mines with great success. The British Commander-in-Chief Sir Douglas Haig decided that the capture of the all important ridge shall be accomplished by mines too.

Over the next several months, Royal engineers carefully dug some two dozen tunnels under German lines and packed them with 450,000 kg of explosives. They were all fired at the same time at 3:10 am on 7 June 1917, creating one of the largest non-nuclear explosions in history. At Lille University's geology department, 20 km away, the shock wave was mistaken for an earthquake. Tremors were also detected by seismographs near Utrecht, at 200 km distance, and on the Isle of Wight, nearly 300 km away. It was rumored that the sound of the explosion was heard as far away as London and Dublin. Eyewitnesses closer to the scene of action reported seeing “pillars of fire” rising out of the ground.

British journalist Philip Gibbs later reported the incident:

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 there were not killed outright. Many of them lay dead in the great craters opened by the mines.

Graphics by BBC

One of the mines under Hill 60 was charged with 24,000 kg of explosives, and another mine under Caterpillar was charged with 32,000 kg of explosives. Both explosions blew away large part of the hill. In total, about 10,000 German soldiers were killed or went missing in the Messines Ridge explosions alone.

On one of the highest points of the Messines Ridge, under Spanbroekmolen, some 41,000 kg of explosives were packed. The mine at Spanbroekmolen was located 27 meters below ground, at the end of a tunnel that began more than half a kilometer away from inside a small wood. The original tunnel was discovered by the Germans and destroyed in March 1917, forcing the Royal Engineers to start a new branch. After three months and only a few hours to go before the scheduled explosion, workers charged the mine and the primer charge and tested the circuits. Finally at 3:10 am, it was detonated along with all mines at Messines. The Spanbroekmolen mine exploded 15 seconds late, by which time the German soldiers had already been ordered to go over the top, and had left their trenches and begun to move across no-man's land. But the blast was so powerful that flying debris killed many British soldiers who stood hundreds of meters away. The blast left a crater approximately 75 meters (250 feet) in diameter, and 12 meters (40 feet) deep. It was this crater that eventually became the “Lone Tree Crater” or the “Pool of Peace”.

The evening before the attack, General Sir Charles Harington, Chief of Staff of the Second Army said to the press: “Gentlemen, I don’t know whether we are going to make history tomorrow, but at any rate we shall change geography.”

Indeed, the explosions at Messines Ridge created several permanent craters that have now turned into shallow pools.

Not far is the Peckham Farm Crater, about 240 feet across. 39,400 kg of explosives was used to blow up the mine that was dug underneath the crater. Further north is the Maedelstede Farm crater, a 100 feet across, created by 41,000 kg of explosives. Further up still are the twin craters of Petit Bois, and less than a kilometer north are the three craters of Hollandscheschur Farm. Then south of Spanbroekmolen are the double craters of Kruistraat.

Some of the mines, however, failed to detonate. They still remain like ticking time bombs underneath private land. One blew up unexpectedly in 1955 during a thunderstorm. A pylon was unknowingly erected above the site of the mine. When lightning struck the pylon, it detonated the mine below. The only casualty was a cow.

Related: Lochnagar Mine Crater

Geological sections of a part of the Messines Ridge mine craters.

The Caterpillar crater. Photo credit: ViennaUK/Wikimedia

Factory Farm Crater. Photo credit: Arthur Jordan/Flickr

The crater of the Peckham mine.

British troops in a captured, badly smashed by the British bombardment, German trench on Messines Ridge. June 7, 1917. Photo credit: Imperial War Museum

A general view showing the destruction of Oosttaverne Wood, including the trenches taken by the British during the Battle of Messines Ridge. Seen on 11th June 1917. Photo credit: Imperial War Museum