The German western flanking tunnel (Stollen 1b), in progress since November 1915, was pushed ahead in difficult conditions in a gallery 80cm in height and 150cm wide. Former Pioneer Herman Hoppe recalled in 1969 that they worked without shirts and had to use oxygen cylinders at the face because the air was so bad. By mid-February the gallery was 86m in length and the Germans prepared a chamber 5m long, 4.5m wide and 2.7m in height to take a massive 50 tonne charge. Five railway trucks were needed to bring up the explosives. It was the largest mine charge of the entire war, larger than the 43.2 tonne St Eloi mine blown by the British at the Battle of Messines in June 1917. Charging began on 25 February and, with tamping, took thirty men three and a half weeks. Hoppe described how the yellow powder of the Westfalit and Astralit explosive came in packages about 25cm long, which they stacked like bricks. Five hundred detonators were used, with three separate sets of leads. The tamping included a 6m barrier of concrete and extended for 80m in total, almost the entire length of the tunnel, requiring 25,000 sandbags with wooden barriers every 5m.¹⁸

The Germans fired the mine on 14 May 1916. Hoppe remembered the moment of detonation:

… the whole mountain rocked. One could feel it at Varennes and beyond like an earthquake. With the debris a leg with the buttocks came over us flying through the air. In my diary it says that four Frenchmen were thrown over which I am not sure about now. But I definitely saw the leg.¹⁹

The crater was almost 60m in diameter, destroying the French lines for 35m right back to the third line and pulverising the French underground shelters and their occupants. One hundred and eight men were missing, mostly from the 9th Company of the 46th Infantry Regiment. Three mine shafts and galleries disappeared and with them nine miners.

It is an upheaval so beyond measure, such a complete eradication, such an absolute transformation, as to seem at first too powerful to enter familiar eyes. I do not comprehend, I cannot. This emptiness crushes me. (Lieutenant André Pézard)²⁰

Despite the massive destruction that the mine had wrought on the French positions, the Germans did not coordinate this mine with any attempt to force the French from the ridge. The only explanation for this is that the Germans wished to secure their hold on the ridge, but it nevertheless suited them to allow the French a small foothold. Two days after the blow General Halloin, commanding the French 5th Corps, issued orders that it was essential to give more power and energy to mining at Vauquois, to catch up with, or even overtake, the Germans and that it should be possible to blow 100 tonnes at the same time with combined charges. The commander of Company 5/1 studied a project to open a breach of 130m in the German lines in the centre of the ridge, to gain observation over the northern slope and to destroy the deep German underground shelters. He planned four charges laid in galleries dug using compressed air-powered drills: two 30 tonne charges would be placed at 25 and 23m depths in two branches from the E4 gallery. Two others of 10 tonnes and 20 tonnes would be at 25m depth at the head of E18. The plan was supported by General Valdant, commanding 10th Division, but would require 250 workers, who were not available. The scheme was abandoned.

French High Command, on the contrary, prescribed a purely defensive attitude to mining, while acknowledging that energetic action was needed to combat the deeper German galleries. Therefore the creation of a second system at a lower level was ordered and the engineers began six new attack galleries from shafts about 30m behind the front line. These had to be inclined downwards at 45 degrees to reach a depth of 30m by the time they passed the French front line. The engineers had to rely on less skilled infantry pioneers to carry this out, whose work was slower. They also had to keep the shafts in the front line in operation, where the fight with the Germans continued unabated. Daily bombardments by well-observed German mortar fire impeded progress by frequently blowing in the entrances of galleries. In May the French tackled this problem by constructing new concealed entrances linked to the front line workings some 15m back.

From April 1916 the Germans deepened their mine system yet further by driving three tunnels, Ost, Mittel and West, at 42m depth into the steep northern slope. They blew a large mine on 20 July, increasing the crater of 14 May in size and further threatening the French galleries in the west. In the east, on 7 June, the French infantry had felt three jolts which they suspected were due to German underground blasting. Lieutenant Pézard described their desperate desire to leave Vauquois; it was a wish that was fulfilled fifteen days later when the 46th Regiment left the ridge for good. The Germans blew another large mine in the east on 26 July, creating a 50m-wide crater which overlapped that of 3 March.

In August 1916 the three French engineer Companies were relieved after over a year at Vauquois, during which time they had lost 215 men killed, and were replaced by Companies 27/51, 16/3 and 27/1. Company 27/51 blew a charge of 10 tonnes gunpowder and 1 tonne dynamite on 19 August, creating a 40m wide crater, although the Germans had detected the work and evacuated their front line and shelters. Over the following three days 1/Pi 30 blew five charges in return. In September Captain Mathey, commanding the 27/1, proposed creating a third system of galleries, which started on the French rear slope, in imitation of the Germans. Three inclined galleries, X, Y and Z, were begun which would pass beneath the French front line at 38m depth. At the end of September the French got electric power and compressed air to these three galleries, allowing the use of pneumatic picks and electric winches. A 40cm rail line was installed on 19 November. These measures went some way towards compensating for the French lack of labour, but the power supply was insufficient for all winches and they had to be used alternately.