"During World War I miners often dug tunnels, and placed mines in them. The main objective of these mines was to destroy part of an enemy trench, and then attack during the confusion. Soldiers eventually developed strategies for discovering enemy tunnels. One method was to put one end of a stick in the ground, and the other end on your teeth to feel vibrations. Another method was to sink a water-filled oil drum into the floor of the trench, and lower your ear into the water to listen to noise being made by people tunnelling.
It took a long time to dig a tunnel. Sometimes it took a year. Occasionally miners would dig into their enemy's tunnels, starting an underground fight.
Typically there would be 4 tunnellers working at the face (one of whom would be resting), plus 2 tunnellers mates handling trolley, air pumping and preparation of timber. Generally infantry working parties were employed to remove the spoil from the shaft heads and dispose of it. (One foot run of subway created 70 sandbags of spoil). At any time a Tunnelling Coy would have several digging parties working towards each other."