A typical mining position, such as would be prepared on an active sector of the front, would consist of an upper level having a series of forked galleries, known as “feelers,” with geophone listening-posts at their extremities, and a deeper level, with numerous “fighting branches” projecting from it, to protect the lower flank. Just as the sentries in the trenches strained their eyes to detect any ominous figures in the darkness of No Man’s Land, so the mining sentinels, crouching over their geophones in the headings of dim-lit galleries, strained their ears to catch the faint sounds which gave warning that the enemy was approaching underground. The geophone, which has proved of incalculable value in mining warfare, is an instrument for augmenting small sounds coming through the ground. The American geophone, which is a highly sensitive, extremely simple, and easily portable instrument, is in no sense an electrical device, resembling, rather, the stethoscope used by physicians for testing the lungs. In mining operations two geophones are used, one for each ear, the instruments being so sensitive that the sounds caused by a fly walking on the wooden support of the geophone appear as loud as the tramp of a horse on the floor of a stable. If a sentinel on duty in an underground listening-post caught through his geophone a sound which was more distinct in, say, his right ear than in his left, he gently shifted one of the instruments, inch by inch, until the sound was the same in both ears. Then, by means of a compass, he took the magnetic bearing of a line perpendicular to that passing through the two geophones, which would give the direction from which the sound came. Meanwhile sentries in the other listening-posts were doing the same thing, so that, by the co-ordination of their reports and by triangulation, the enemy’s gallery could be located within a few yards.