As he descended, Malcolm found that the smoke was dispersing under the influence of a steady draught of warm air. The tunnel was heavily timbered--top, sides, and floor. Along one side ran a couple of insulated wires, one of which belonged to an electric alarm-bell. The other was for internal lighting, but every incandescent bulb had been shattered under the terrific concussion of the great Messines mine. In places the massive planks were bulging ominously; so much so that Captain Nicholson hesitated more than once.