Detecting the noises made by the enemy underground was a crucial form of intelligence and miners spent long periods attempting to discern the location and activities of their opponents. Listening became the most important means of gaining information on the activities of the opponent underground. It was also the most nerve-wracking activity of underground warfare, requiring a man to crouch in a tunnel for long periods, often alone, and in close proximity to the enemy. Graffiti surviving deep in the Vimy Ridge system of 172 Tunnelling Company indicates only a small number of men employed on listening, working in pairs, and on duty for many consecutive days in the same chamber. As well as good hearing, only a small number of men possessed the temperament to make effective listeners. The hearing and interpretation of sounds was highly subjective and Capitaine Thobie ranked his listeners according to how reliable they were. He placed the best in galleries where the tangle of surrounding tunnels was most complex, as they had to distinguish between noises from their own adjacent galleries and those of the enemy. In the most dangerous places:

…a kind of fever of anxiety and fear reigned permanently. The listener’s attention was constantly drawn by the workers, to listen to the slightest movement indicating the presence of the enemy miners. At the start of day, all went well; but, as the hours passed, the constant reminder of the burden of responsibility by all those who carried out their task with the greatest possible activity acted on the listeners. Any routine incident of this terrible existence in the trenches 30 or 35 metres from the enemy: a mortar bomb or rifle grenade bursting in the vicinity, all contributed to tightening the nerves and wore down the resolve of the listener towards his task which was to report the slightest noise heard. The result, especially at night where the danger of raids increased, was to excite the listener to the point where he imagined that he heard noises. He warned the workers and the foreman NCO, and as obviously each felt relieved of the individual burden of responsibility of claiming to hear something, there arose a strong conviction of the certainty that noises had been heard which had no basis in reality. These misapprehensions had to be combated in order not to fall into the same error oneself, because a charge, placed without reason at a point where the enemy had not yet reached, led to waste of time and a partial destruction of our own system without any profit.¹⁶

Listening records had to be carefully kept, so that patterns could be detected, and it was important that they were handed on to relieving units. Where there was anxiety over mining, engineers would frequently be asked to investigate sounds by the infantry, who believed that they were being undermined. Often these would be discovered to be harmless activity in an adjacent dugout, or on occasion a nest of rats. Where miners did discover signs of activity, however, they might not reveal to the infantry that the enemy was beneath their trenches, for fear of inducing panic.