 Since October 1916, Royal Engineer tunnelling companies had been working on the Western Front. The Artois region is chalky and easily excavated and under the city of Arras, there is a vast network of boves (caverns, underground quarries, galleries and sewage tunnels). The Third Army at Arras planned to use the old underground quarries in the city in the offensive planned for April 1917. The quarries were to be linked by tunnels so that they could be used as shelters against German artillery-fire and to convey troops to the front in secrecy and safety. Assault tunnels were also dug, stopping a few metres short of the German line, ready to be blown open by explosives on Zero-Day. Conventional mines were laid under the German front line, ready to be blown immediately before the assault. (Many were never detonated for fear that they would churn up the ground too much.) German sappers (military engineers) also conducted underground operations, seeking out Allied tunnels to assault and counter-mine.
