 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.
