In the winter of 1916, a plan for a major offensive was being planned by General Sir Herbert Plumer; its main objective was to take the Messines Ridge on the southern end of the Ypres Salient. This area was of strategic importance and had been held by the German Army since December 1914. In January 1917, work began and over 8,000 metres of tunnels were dug and 21 mines containing approaching 600 tonnes of explosives were laid under the German lines. At 0310 on June 7th 1917, the British Army simultaneously blew nineteen of the mines under the German lines. It was the largest man-made explosion before the nuclear age, which astounded all who witnessed the cataclysmic event, likening it to a huge volcanic eruption.