These cellars and underground quarries formed the basis of the scheme on which the company now embarked. They were to be connected, opened up, and made habitable for troops, so that when the day for attack came the men could issue from them safe, warm and dry, and utterly unsuspected by the enemy. Two series of these caves and underground quarries were discovered stretching towards the enemy lines, one under the suburb of Ronville and one under St. Saveur and the Arras-Cambrai Road.

The Ronville caves were by far the larger, some of them immense caverns hundreds of feet in diameter and twenty to forty feet high. Pillars of chalk had been left at regular intervals to support the roof, but in the centuries that had elapsed since their abandonment, falls had gradually formed them into inverted cones springing away to the domed roof between, so high that it was barely discernable in the candle light.

To make these caverns safe and habitable was no easy matter, for as soon as they were opened to the cold wet winter air the chalk commenced to swell and crack, and slabs weighing many tons would come crashing down without an instant’s warning. To timber up to the heights of these roofs was out of the question, so instead the floor was raised by dumping therein the chalk cut from the galleries and dugouts until the roof was close enough for clear observation and support. (Lieutenant J.C. Neill, New Zealand Tunnelling Company)²⁶

These caves were to accommodate 11,500 troops and the largest, Christchurch, held 4,000. The caves were provided with water and electricity and protected by gas-proof doors. They carried telephone cables, exchanges and line testing-points. A tramline ran from the sewer to the St Saveur caves. Just over half a mile from the front line a cave was equipped as a main dressing station, as it was anticipated by VI Corps that the congestion in Arras during the attack would prevent the evacuation of wounded for treatment, so advanced dressing stations were equipped and manned to perform the functions of a main dressing station. These were established in caves, cellars and the basements of buildings. The chief was close to the 3rd Division trenches, two sloped entrances allowed stretchers to be carried direct from the communication trenches, while an approach at the back allowed motor ambulances to collect casualties. Named Thompson’s Cave after the Corps Director of Medical Services, it was able to accommodate 700 casualties on stretchers fitted in two tiers. It also contained dressing and operating rooms, kitchens and latrines. The accommodation close behind the front line enabled it to act as a corps dressing station for severely wounded requiring urgent attention. The cave was used for the first two days of the offensive until a large shell exploding on the top of the cave burst a water main, which caused the roof to collapse in two places.²⁷

The New Zealanders also constructed two long tunnels towards the front line, one linking the St Saveur cave system and the other the Ronville caves so that men could if necessary get all the way to the front line through the caves and tunnels and avoid especially the heavily shelled area of the railway station. The St Saveur tunnel ended in five exits from Russian saps in no man’s land and the Ronville with one, although others were probably under preparation. About seven miles of tunnels and subways were constructed in just over four months.²⁸