The British drew up guidance on the range of noises in chalk or clay. Picking in chalk travelled the furthest: it could be heard 150ft away with the naked ear and 300ft away with a geophone. Shovelling was audible at 70ft or 120ft with the geophone. Talking was perceptible at 12ft or 50ft with the geophone.¹⁹ In sandy clay the distances were about two and a half times less²⁰ and work in clay was always more liable to break into the galleries of the opposing side. In the chalk areas it was far more difficult to place a mine directly beneath the enemy trenches and most blows tended to be in no man’s land.

Effective listening was usually only possible if work stopped on the listener’s side. This would be done either between shifts or during specified listening periods, when an officer would enter the gallery alone for a period of perhaps half an hour. At Givenchy in 1915, for example, the Germans stopped all work for twenty-five minutes every two hours.²¹ Listeners, especially when using the geophone, had to learn to recognize the sounds of a range of underground activities. Picking and shovelling were the most common sounds heard, along with the knocking in of timbers, the dragging of bags of spoil along the tunnel, or the pushing of a wheeled trolley. Two men picking at once could indicate that a gallery was being enlarged to form a charge chamber. The dragging or tramming of bags up the gallery to the face would indicate explosives, followed by the further bringing up of bags of spoil for tamping, possibly accompanied by the knocking out of timbers being salvaged from a gallery to be blown. Most ominous and threatening for a listener, after a period of such activity, was silence. This indicated that a charge was laid and tamped and that the enemy was waiting to blow. To disguise this tell-tale silence, miners would continue with false picking in an adjacent gallery until the last minute before blowing, to persuade the enemy that it was safe to remain below. Both sides used dummy picks, operated with a long rope, and the British captured one in the German mining store in Fricourt. It is unlikely, however, that these subterfuges fooled experienced listeners. Another means of deception was the use of work on two levels, used, most famously, by the Australians at Hill 60 to disguise any sounds from the deep Berlin Tunnel. Occasionally, when miners came up against the timbers of an enemy gallery, they were able to carry on protracted listening in an effort to gain some useful intelligence. In June 1916, after they broke into a German shaft head, the British recorded the conversation of German miners for four days, but little of interest was learned.²²

The danger of poisonous or flammable gas released by explosions was seriously underestimated by both sides in 1915. As charges grew larger in size, so the quantities of gas, in particular carbon monoxide and methane, that were released by explosions underground increased. Carbon monoxide is colourless and odourless and as well as flooding mining systems could also flow up a shaft to gas men on the surface, even causing them to collapse and fall down the shaft. Many succumbed before they were aware, while others might suffer a headache and drowsiness or giddiness. It also caused temporary or permanent mental aberration. Often men became argumentative or violent, physically attacking their rescuers, or hysterical and childish. The mental confusion might last for many weeks. A British officer who was rescued unconscious came round after several hours and was quarrelsome, shrieking and swearing. Later he had no recollection of his accident and was oblivious to his surroundings. After six months his mother told doctors how she had to help him on his bad days and remind him how to carry out simple tasks such as tying his shoelaces.²³