Over the winter months, the 170th Tunnelling Company RE dug several galleries under the German lines, in the Redoubt area, which had changed hands several times since September 1915. In March 1916, the Germans had an unobstructed view of the British positions, from a slag heap named "Fosse" 8 and in previous mining operations, no man's land had become a crater field. The British front line was held by outposts, to reduce the number of troops vulnerable to mine explosions and the strain of knowing that the ground could erupt at any moment. The 12th (Eastern) Division was selected to conduct an attack, intended to capture the crater field, gain observation from crater lips over the German defences back to "Fosse" 8 and end the threat of German mine attacks.