Long before the engineers came with their shafts and tins, Spanbroekmolen was exactly what its Flemish name said: the windmill at Spanbroek, a few farmhouses gathered on one of the highest points of the Messines Ridge, looking out over the villages of Kemmel, Wijtschate and Wulvergem. A windmill had stood on this rise for three centuries. The Germans wrecked it on 1 November 1914, in the first weeks of the war, and never let it be rebuilt — the ground was too useful as a strongpoint. From here, German observers could see deep into the British rear. The Allies were looking up at the ridge from the wet ground below. Whoever held Spanbroekmolen could see everything. Whoever did not held no real position at all.

Because the ridge could not be taken in any ordinary way, the British decided to go under it. In December 1915, the 250th Tunnelling Company sank a sixty-foot shaft into the Flanders clay and began driving a gallery toward the German line at Spanbroekmolen. Over the next eighteen months the work passed from one tunnelling company to the next — the Canadians, the 175th, finally the 171st — as men inched the chamber forward in candlelit silence, listening for German counter-miners doing the same thing from the other direction. In March 1917 the Germans found the gallery and blew it in from their own Ewald shaft, cutting the explosive charge off behind a wall of collapsed earth. The British started a parallel tunnel and broke back through. Mining gas overcame several of the miners. Only hours before the attack, officers used torch batteries to test the firing circuit.

Spanbroekmolen was one of nineteen mines fired beneath the Messines Ridge that morning in June 1917 — the largest planned explosion in history up to that point. The shockwave was felt in London. German fortifications across the ridge ceased to exist; survivors who emerged from what had been their front line surrendered in a daze. The Battle of Messines, brief and almost surgical by the standards of the Western Front, ended with the New Zealand Division taking the village of Mesen and the British Second Army holding ground that had been German since 1914. Some of the men killed by debris falling back from the Spanbroekmolen blast — including British soldiers caught by their own explosion — are buried at Lone Tree Cemetery just to the south, where a single tree once stood on the lip of the new crater.

After the war, the crater filled with rainwater and the farms came back. The hole could easily have been levelled, or built over, or forgotten. In 1929, Charles Cheers Wakefield — Lord Wakefield, the oil magnate behind Castrol — bought the crater outright and gave it to the Toc H movement, the Christian fellowship founded by Tubby Clayton at Poperinghe during the war, on the condition that it be preserved exactly as it was. Trees were planted around the rim. A small memorial was set into the grass. He renamed it the Pool of Peace. The naming was an act of deliberate inversion: the deepest hole anyone had yet blown in the earth, made over as a quiet pond. The water is dark and still. Frogs live in it. The ridge is silent.

Two cemeteries sit within sight of the water. To the south is Lone Tree. To the northeast is the Spanbroekmolen British CWGC Cemetery, holding men of the Ulster and Royal Irish divisions who fell in the attack the mine opened. Between them, on the ridge itself, there is a small stone for Lieutenant Cyril Gordon Martin, who earned the Victoria Cross here on 12 March 1915 — wounded before he started, he led his bombing party into a German trench at Spanbroekmolen, held it for two and a half hours against counter-attack, and walked out alive. He went on to be a brigadier in the Second World War. The crater outlived him too. It will probably outlive everyone who comes to stand at its edge.