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. "Yes," rejoined Walt with a shudder, "and but for Providence, we should have plunged downward into it ourselves." "Ugh!" exclaimed Jack, in a voice filled with repulsion. "Don't let's think of it. See, the path takes a turn here. Come on, let's go ahead, but follow me closely and keep in to the wall." "Not likely to take any chances of missing the road, after seeing that," spoke up Walt, as once more the three youths, who had been so strangely plunged into this predicament, began to tread the subterranean regions once more. As you may imagine, they went with due caution. But no more dangers menaced them, and as they progressed the path began to widen. All the time, however, the strange roaring sound had been growing louder, until now it had attained almost deafening proportions. Still they had come upon no explanation of what it could be. Jack had privately concluded it to be the sound of the wind, forcing its way into some crevice. This theory seemed to be the more tenable as the last match which he had struck had only been kept alight with difficulty, so strong had been the draught that now puffed up toward them.