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.

At their very feet--so close that the tips of their boots almost
projected over it--was a deep chasm. The black profundity of it loomed
in front of them gapingly. A few paces more, and they would have been
precipitated into the abyss. Jack, suppressing a shudder, leaned
forward and held the match as far over the edge as he dared. As the
depths of the great crevasse were illuminated by a feeble flame, he
shrank back with a sharp intake of his breath.

[Illustration: As it flared up, they all three recoiled with
expressions of dismay. At their very feet was a deep chasm.]

The place was a charnel house!

No mystery now as to what had become of the human remains of the grisly
sacrifices of the ancient mesa dwellers. There, piled in that dark
chasm beneath them, were great piles of decaying bones and gleaming
skulls. Hundreds of them extended toward the surface in a ghastly
pyramid. No wonder the underground place into which they had
penetrated smelled musty and unpleasant.

"It is the mesa dwellers' burial ground!" exclaimed Ralph in a
quavering voice, as, clinging to Jack's arm, he bent forward.