ies into one of those
queer underground rivers we know are to be found all through the
Southwest. And Smith ought to know something about it, for he's been
watching this business a whole year now, from close quarters."

"I'm willing to take the chances, if you are," declared Bob.

Frank was not at all surprised when he heard his chum say this. He
knew that the Kentucky boy was apt to be rash; and that meant more
caution on his part, in order to counteract this spirit, that might
border on recklessness.

A quick decision had to be made, for delay could do them no good. He
cast one last look up at the dark heavens, as though questioning how
long they might remain mute.

"All right, we'll risk it, Bob," he declared, suddenly; for even if the
worst came Frank believed he knew how to avoid a calamity.

"Good for you, Frank!" exclaimed the other; but Bob understood the
nature of the risk they were taking, and he was not quite so buoyant as
usual.

The canyon was just below them now, and fortunately there seemed a
narrow bit of slope down which they might make their way. This they
did with considerable difficulty. Indeed, Bob was secretly sorry,
after they had started, that he had urged his companion to take this
step; but there could be no going back now.

Finally, after several slips, and more or less excitement, they managed
to gain the bottom of the canyon.

"Say, I don't remember this place any, Frank!" declared Bob, as he
stared about him as well as he could by the flickering light of the
lantern which his companion still carried, and which had served them
well through all their descent.

"For a mighty good reason," replied Frank. "We were never here before."

"But this is the same old turtle crawl, isn't it; the _barranca_ we
followed up to the time we climbed the slope with our horses?" Bob
asked.

"It sure is, only a lot farther along, Bob. Notice how the walls tower
upon each side. I knew something about this, and that was why I held
back when you wanted to come down here. But let's hurry. We've got to
make that slope as soon as we can."

"Supposing the thing broke loose before we could find any place to
climb out?" suggested Bob, looking up again with awe, as he stumbled
along after his chum, who was already hurrying down the canyon.

"We might try to outrun it first," Frank replied, over his shoulder.

"And if that didn't work, what then?" the other continued.

"Nothing left but to clim