The attack effectively began on 3rd June when the preliminary bombardment intensified, and was kept up until 0250 hours on 7th June. By this time, 100,000 men of the Second Army were lying in position waiting to attack. The weather was clear with a bright moon. The sudden silence spooked the Germans who started firing flares in an effort find an explanation. Twenty minutes of tension-packed waiting culminated in a loud bang, followed seven seconds later by a continuous series of huge explosions which tore at the German front line and threw the watching British, 400 metres away, off their feet.
The British rose from their trenches under cover of the renewed barrage of every gun available. Nine divisions of infantry advanced through the clouds of smoke and dust and within minutes, the whole of the German front line was in British hands. Three hours later, the whole of the Messines Ridge was taken. No official figures were ever released regarding German casualties but there were 7,354 prisoners taken. There were 10,000 reported missing and over 6,000 known dead. British casualties numbered 16,000 of which about thirty per cent were killed.
The success of the assault was in large part due to the explosion of 19 mines tunnelled under the German front line. Preparation work started in 1915 but it was only in the winter of 1916 that serious preparations took place. Twenty-two mines were dug, some up to 2160 feet (658 metres) long and up to 125 feet (38 metres) deep. One mine (at Petite Douve Farm) was discovered by German counterminers on 24th August 1916 and destroyed. Two mines close to Ploegsteert Wood were not exploded as they were outside the attack area. The explosion was heard by Lloyd George, the British Prime Minister who was in his study in 10 Downing Street in London, and there is even a report of an insomniac student hearing it in University College, Dublin.
The nineteen mines were located as shown below:
Name of Mine Charge Crater Diameter Dug By (lbs) Hill 60 A 53 500 191 feet 1st Australian Tunnelling Coy. Hill 60 B 70 000 260 feet 1st Australian Tunnelling Coy. St Eloi 95 600 176 feet 1st Canadian Tunnelling Coy. Hollandscheschour 1 34 200 183 feet 250 Tunnelling Coy. Hollandscheschour 2 14 900 105 feet 250 Tunnelling Coy. Hollandscheschour 3 17 500 141 feet 250 Tunnelling Coy. Petit Bois 1 30 000 175 feet 250 Tunnelling Coy. Petit Bois 2 30 000 217 feet 250 Tunnelling Coy. Maedelstede Farm 94 000 217 feet 250 Tunnelling Coy. Peckham 87 000 240 feet 250 Tunnelling Coy. Spanbroekmolen 91 000 250 feet 171 Tunnelling Coy. Kruisstraat 1 } 30 000 Kruisstraat 4 } 19 500 235 feet 171 Tunnelling Coy. Kruisstraat 2 30 000 217 feet 171 Tunnelling Coy. Kruisstraat 3 30 000 202 feet 171 Tunnelling Coy. Ontario Farm 60 000 200 feet 171 Tunnelling Coy. Trench 127 Left 36 000 182 feet 3rd Canadian Tunnelling Coy. Trench 127 Right 50 000 210 feet 3rd Canadian Tunnelling Coy. Trench 122 Left 20 000 195 feet 3rd Canadian Tunnelling Coy. Trench 122 Right 40 000 228 feet 3rd Canadian Tunnelling Coy.
The two unexploded mines were planned to be dismantled by the British but with the impending start of the Third Battle of Ypres, there was always something else to do. When the Germans launched their Lys Offensive in April, 1918, the British HQ was overrun and the documents relating to these two mines was lost and they never were dug up. The precise location of them was not known and they were forgotten until during a thunderstorm on 17th July, 1955, one of them exploded. No one was killed but the explosion did some slight damage to some distant property. The other mine is still, as far as anyone knows, still lurking under the Flanders countryside.
I have gleaned this information from sources to numerous to mention but I am very grateful for their efforts. One source which should be noted especially is:
War Undergroud - The Tunnellers of the Great War, by Alexander Barrie, (ISBN 1-871085-00-4) where the story of this endeavour is dealt with in great detail.