In January 1917, General Plumer gave orders for over 20 mines to be placed under German lines at Messines. Over the next five months more than of tunnel were dug and 450–600 tons of explosive were placed in position. Simultaneous explosion of the mines took place at 3:10 a.m. on 7 June 1917. The blast killed an estimated 10,000 soldiers and was so loud it was heard in London. The near simultaneous explosions created large craters and ranks among the largest non-nuclear explosions of all time. Two mines were not ignited in 1917 because they had been abandoned before the battle, and four were outside the area of the offensive. On 17 July 1955, a lightning strike set off one of these four latter mines. There were no human casualties, but one cow was killed. Another of the unused mines is believed to have been found in a location beneath a farmhouse, but no attempt has been made to remove it. The last mine fired by the British in World War I was near Givenchy on 10 August 1917, after which the tunnelling companies of the Royal Engineers concentrated on constructing deep dugouts for troop accommodation.