text: Underground After the Allied conference at Chantilly, Haig issued instructions for army commanders on 17 November 1916, with a general plan for offensive operations in the spring of 1917. The Chief engineer of the Third Army, Major-General E. R. Kenyon, composed a list of requirements by 19 November, for which he had 16 Army Troops companies, five with each corps in the front line and one with XVIII Corps, four tunnelling companies, three entrenching battalions, eight RE labour battalions and 37 labour companies. Inside the old walls of Arras were the Grand Place and Petit Place, under which there were old cellars, which were emptied and refurbished for the accommodation of 13,000 men. Under the suburbs of St Sauveur and Ronville were many caves, some huge, which were rediscovered by accident in October 1916. When cleared out, the caves had room for 11,500 men, one in the Ronville system housing 4,000 men. The  Crinchon sewer followed the ditch of the old fortifications and tunnels were dug from the cellars to the sewer.
  Two long tunnels were excavated from the Crinchon sewer, one through the St Sauveur and one through the Ronville system, allowing the 24,500 troops safely sheltered from German bombardment to move forward underground, avoiding the railway station, an obvious target for bombardment. The St Sauveur tunnel followed the line of the road to Cambrai and had five shafts in no man's land but the German retirement to the Hindenburg Line forestalled the use of the Ronville tunnels, when the German front line was withdrawn  and there was no time to extend the diggings. The subterranean workings were lit by electricity and supplied by piped water, with gas-proof doors at the entrances; telephone cables, exchanges and testing-points used the tunnels, a hospital was installed and a tram ran from the sewer to the St Sauveur caves. The observation post for the VI Corps heavy artillery off the St Sauveur tunnel, had a telephone exchange with 750 circuits; much of the work in this area being done by the New Zealand Tunnelling Company.