The air had pulsed to the faintest sound; now all was still again. "What was it?" I asked, my voice a whisper. "Don't know, Bill. Haven't an idea. There!" Again that gentle pulsation touched the ear, and again it was gone. And a strange thing was that, for the life of me, I could not have told whether it came from below or from behind us. "There it is again!" said Rhodes. I flashed on my electric light, to the full power. "A whisper!" I exclaimed. "Angel, demon, human or what? And, great Heaven, Milton!" "What now, Bill?" he asked quickly. "It's something _behind us_!" He started. He turned his light up the tunnel, and for some moments we stood peering intently. Not a moving thing was to be seen there, however--only the moving shadows. "Again!" said Milton Rhodes. "But it isn't a whisper, Bill. And it didn't come from up there." "The thing," I told him, "could be hiding in shadow--hiding and watching us." "It's not up there; it's ahead." "Wherever it is, what on earth can it be? Whatever it is, what does this mean?" "That we shall learn." We resumed our descent, every sense, you may be sure, on the qui vive. The tunnel here inclined rather steeply; a little space, however, and the dip was a gentle one. The sounds soon became one steady, unbroken whisper; then a dull melancholy murmur. Abruptly Milton Rhodes stopped. He turned to me, and he laughed. "Know now what it is, Bill?" This was not a moment, I thought, for laughter or anything like it. "Sounds like the growling of beasts," I said, peering intently down the passage. "I wonder if the angel--there are two kinds of angel, you know--has turned loose a whole pack, or herd, or flock, of those demons." To my surprise and astonishment, Rhodes burst into outright laughter. "Well?" said I rather testily. "Why all the cachinnation?" "Forgive me, Bill. But it isn't a pack of demons--or a flock of those charming creatures." "How on earth do you know _what_ it is?" "It's water." "Water?" "Yes. H_{2}O." "Water? I'm from Missouri. You'd better see that your revolver is handy. Who ever heard water make a shivery sound like that?" "You'll see that I'm right, Bill, though I think that you'll hear first." Ere long there could be no doubt about it: Milton was right; it was the sound of falling water. I was not in a hurry, however, to admit the fact. I had to let myself down gracefully. At length, though, it was impossible to hold out any longer. "Must be at quite a distance," I said; "sounds carry a long way in tubes, and that is what this tunnel is." Steadily we made our way along and down, and, just as steadily, the sound increased in volume. The gallery made several sharp turns, and then of a sudden the sound rose from a loud growl to a roar, and we fetched up and an exclamation burst from us.