 The British had begun a mining offensive against the German @-@ held Wijtschate ( sic ) salient in 1916 . Sub @-@ surface conditions were especially complex and separate ground water tables made mining difficult . To overcome the technical difficulties , two military geologists assisted the miners from March 1916 , including Edgeworth David , who planned the system of mines . Sappers dug the tunnels into a layer of " blue clay " 80 – 120 feet ( 24 – 37 m ) below the surface , then drifted galleries for 5 @,@ 964 yards ( 5 @,@ 453 m ) to points deep underneath Group Wytschaete 's front lines , despite German counter @-@ mining . German tunnellers came within metres of several British mine chambers , found the mine at La Petite Douve Farm and wrecked the chamber with a camouflet . The British diverted the attention of German miners from their deepest galleries , by making many secondary attacks in the upper levels . Co @-@ ordinated by tunnelling companies of the Royal Engineers , Canadian , Australian , New Zealand and British miners laid 26 mines with 447 long tons ( 454 t ) of ammonal explosive . The British knew of the importance the Germans placed on holding the Wijtschate salient , after a captured corps order from Gruppe Wijtschate stating " that the salient be held at all costs " was received by Haig on 1 June . In the week before the attack , 2 @,@ 230 guns and howitzers bombarded the German trenches , cut wire , destroyed strong @-@ points and conducted counter @-@ battery fire against 630 German artillery pieces , using 3 @,@ 561 @,@ 530 shells . 
