There came at length a rough wooden door, which swung easily open, and beyond a much narrower and higher passage than before, a more natural cleft in the immemorial rock, it seemed, owing nothing to the agency of human hands. It dripped with water. Hitherto I had been walking on a level, now I trod a fairly steep descent, while the path was no longer straight, but full of fantastic twistings. Each moment the air grew cooler, and each moment a deep, murmurous noise, like very faint and muffled drums, grew louder. The lights, now suspended from a thick and tarry cable, were less frequent than at first, and the place was full of shadows. But as for the noise, that could only be one thing, the Atlantic ground-swell. I was approaching the sea, doubtless by one of the old mine "adits," made for ventilation many years ago and long before the invention of the electric fan. The narrow way ended in a door. It was latched but not locked, and I pushed it slowly open. Immediately there was a sense of vast and gloomy space. I say "gloomy," for it was not absolutely dark. Here and there hung dim, yellow lights.... Advancing a step or two upon a floor of hard earth on sand, I found myself in a vast cavern. It seemed as large as the shell of a cathedral, and for organ there was the plangent, echoing sound of sea waves. The sound came from my right, and was carried on a current of sweet, brine-laden air. Peering through the darkness, I seemed to be aware of a faint, ghostly radiance, a considerable distance away. I had lost the capacity for amazement, but not of quick thinking. In a lightning flash of realization I knew that I had penetrated to the heart of Helzephron's secret, even before my thoughts arranged themselves in sequence. And then, as near as possible coincident with my stepping through the door, I heard a shout. Someone had seen me.... The shout came from the other side of the long, aisle-shaped cave. Simultaneously, half-way up the side, at a height of thirty feet from the floor, there was a sudden illumination. I saw a broad ledge in the wall, railed round, with a ladder staircase descending from it. A little black figure was leaning over the rail, and it was from this that the shouting came. It did not need his words to tell me that here was a wireless station. I could see the drum and the battery shelf quite distinctly. "A signal!" he shouted, and I knew that he took me for the dead man above. "They're coming back! The sky swarms with armed patrols and warships. They've had to run for it, but the Chief thinks he's shaken them off. I must switch on the guides!" I gave an answering shout, keying my voice down to something like Feddon's bass growl. "It's C.Q.D.!--C.Q.D.!" came in a shrill voice of alarm, and Mr. Vargus ran down the ladder like an ape. C.Q.D.! The signal of "extreme danger." Well, I rather thought it was!