"Say," cut in Jack briskly, "if it was light enough to see, I'd give you a good licking. Doomed to die, indeed! Not much. It's a cinch, isn't it, that if there is an entrance to this place there must be an outlet, too? Very well, then," he hurried on, without waiting for an answer, "let's find that outlet." The logic of this speech might be questioned, but of its good sense, under the circumstances, there was no doubt. "You're right, Jack," said Ralph. "I'm ashamed of myself for doing the baby act. Come on, let's set out at once." "That's the talk," said Walt heartily; "if there's a way out, we'll find it." "And if not?" asked Ralph, his spirits flagging again. "We'll discuss that later," declared Jack briskly. Returning again to the landing--if such it might be called--upon which they had terminated their abrupt descent into the interior of the mesa, some more of the precious matches were lit. As the last flickered out, the boys fancied that some feet from them they could see a black mouth, like the entrance of a tunnel, or rather a continuation of the one into which they had been thrown. "Come on, boys," exclaimed Jack. "It's the only thing to do. We can't turn back, and, as Pete says, 'there ain't nothing to do but go ahead.'" Not without some misgivings did the three lads plunge forward in the darkness, feeling their way with outstretched hands as they entered the tunnel. A close, musty smell, as of things long mildewed and moulded, filled the air, and an oppressive silence lay on everything. Unconsciously, since entering this place, their conversation had been all in whispers. The tunnel they were now traversing was bored on a pretty steep down grade. So steep, in fact, that Jack concluded, after about a quarter of an hour of slow and cautious traveling, that they must be below the level of the desert. For the last few minutes they had been conscious of a peculiar thing. This was that the silence of the tunnel had given place to a deep-throated roaring, not unlike the voice of a blast furnace. Where it came from, or what it was, they had no idea. It was a most peculiar sound, though, steady as a trade-wind, and seeming to fill the whole place with its deep vibrations. "What can it be?" gasped Walt, as they paused by common consent to listen. "Maybe the wind roaring by the entrance to this place," suggested Jack hopefully. This thought gave them new courage, and, on Ralph's suggestion, Jack struck another match from his store. As it flared up, they all three recoiled with expressions of dismay.