The Canadian Corps was posted to the northern part of Vimy Ridge in October 1916 and preparations for an attack were revived in February 1917. British tunnelling companies created extensive underground networks and fortifications. Twelve subways, up to long were excavated at a depth of and used to connect reserve lines to front lines, permitting soldiers to advance to the front quickly, securely and unseen. Often incorporated into subways were light rail lines, hospitals, command posts, water reservoirs, ammunition stores, mortar and machine gun posts and communication centres. The Germans dug a number of similar tunnels on the Vimy front, to provide covered routes to the front line and protection for headquarters, resting personnel, equipment, and ammunition. The Germans also conducted counter-mining against the British tunnellers and destroyed a number of British attempts to plant mines under or near their lines.