I had seen the working of the tunnelers up by Hill 70 and elsewhere. I had gone into the darkness of the tunnels, crouching low, striking my steel hat with sharp, spine-jarring knocks against the low beams overhead, coming into galleries where one could stand upright and walk at ease in electric light, hearing the vibrant hum of great engines, the murmur of men’s voices in dark crypts, seeing numbers of men sleeping on bunks in the gloom of caverns close beneath the German lines, and listening through a queer little instrument called a microphone, by which I heard the scuffle of German feet in German galleries a thousand yards away, the dropping of a pick or shovel, the knocking out of German pipes against charcoal stoves. It was by that listening instrument, more perfect than the enemy’s, that we had beaten him, and by the grim determination of those underground men of ours, whose skin was the color of the chalk in which they worked, who coughed in the dampness of the caves, and who packed high explosives at the shaft-heads⁠—hundreds of tons of it⁠—for the moment when a button should be touched far away, and an electric current would pass down a wire, and the enemy and his works would be blown into dust.