Third Battle of Ypres
For 18 months the British prepared 21 huge land mines underneath the German lines along the Messines Ridge southeast of Ypres, Belgium, a location that had already seen major battles in 1914 and 1915. The British detonated these massive mines in early June 1917, though only 19 exploded. One exploded in the early 1950s, and one remains unexploded under the ridge (no one knows its exact location). This was to enable the British to gain the heights. The battle began on July 31st, when the British forces charged the German positions.
At first the strategy worked, as the Germans were confused and disorganised by the preliminary bombardment. But the British did not pursue the Germans as quickly as they should have. Rain began to drench the area in one of the wettest summer and autumn period in years, in a low-lying area where years of fighting had destroyed the land drainage system. Soon the Allied forces became bogged down in the mud, and the British only managed to gain a few miles during the following months, finally gaining the ridge of Passchendaele before winter forced an end to the fighting. For the loss of several hundreds of thousands of men , the British achievement was to gain the heights around the Ypres Salient, which meant the area became relatively safer - at least for a few short months until the German Spring Offensive of March, 1918.