Suddenly the listeners, crouching in their hiding-place, heard a shout of astonishment from the Mexicans.
"They've seen our camp over the edge of the mesa!" exclaimed Pete in a low, tense voice; "in another minute they'll start looking for us."
As he spoke, the voice which Jack had recognized as Black Ramon's, uttered a crisp, curt command of some sort. The lads could hear footsteps hurrying hither and thither. Without doubt, the order that meant their probable doom had just been given.
"I can't stand this a minute longer," cried Ralph suddenly. The boy's eyes were blazing wildly. Clenching his fist, he sprang to his feet.
"Come back here, you blockhead," snapped Jack, tugging his friend down. Ralph came backward sprawling, and landed in a heap in Jack's lap, knocking Walt Phelps with him. Together the three boys were tangled in a struggling heap.
"Get up," whispered Jack. "They'll hear us. You——"
He stopped short. All at once an astonishing—an incredible thing—had happened. The floor beneath them,—the solid floor, as it had seemed,—began to tremble.
Before any of the amazed lads could utter a word, the foundation upon which they rested tipped, and, with a loud, ringing cry of terror from Ralph, they were plunged out of the sunlight into blackness as impenetrable as the pocket of Erebus.