The best way for tunnellers to locate the enemy underground during the First World War was to use the geophone. The New Zealand Tunnelling Company official history tells us that the geophone:
consisted of a pair of wooden discs about four inches in diameter by a inch and a half thick, in the centre of which was a layer of mercury contained between mica plates: these were connected by rubber tubes to stethoscopic earpieces. In use the two discs were placed in contact with the ground and the listener knelt in front of them with ear-pieces adjusted. By moving one disc in an arc round the other until a particular sound was heard with exactly equal intensity in each ear, the direction of the sound was located in the line at right angles to that between the centres of the two discs, a compass bearing of which could be taken.
By making this observation from two or more widely separated points and plotting the results on paper the intersections of the bearings would give the exact location. A simple and wonderfully efficient little instrument, provided the user had sufficient experience and was not too plentifully endowed with imagination. When it is understood that in listening with it in the solid chalk of our front, practically every sound within a radius of 300 feet was distinctly audible it will be realised that a lively imagination can produce some weird effects in the listening reports: one classic sample distinctly heard a horse munching oats at 100 feet below the line – it could only have been a pre-historic fossil one!
To kneel or sit for hours at the end of a narrow gallery out under no man's land, in bad air, with only a guttering candle as protection from the ultimate dark, with every faculty concentrated on the sense of hearing alone to pick up the faint tap-tap of a Hun miner's pick, to separate that sound from the innumerable others, men walking on the trench boards far overhead, a sentry kicking his numbed feet against a firestep, the crash of a "Minnie" [German trench mortar] or the rattle of a machine gun, or even the scurryings and love affairs of the trench rats: to keep concentrated when it stops (the Hun is a very intermittent pick man), and to pick it up again the instant it starts, and then to determine exactly in which ear the sound is the stronger and to know that perhaps on this knowledge depends not only your own life and that of your mates but also the lives of those patient infantry in the trench above, all this will be summed in the laconic official listening report "Enemy picking Intermittent Faint 18 deg." Or again, to listen through the night to the stealthy shufflings and dragging noises that indicate the enemy is charging and tamping his mine, to determine, when the last faint rustle ceases, that he is ready to blow and so warn the line; all this is the listener's job and perhaps there was no other that strained body, brain and nerve as did this.