Far from alarming them, however, this gave them renewed hope. It meant that, in all probability, they were nearing an outlet of the strange underground place. Had it not been for the predicament in which they had left the professor and Coyote Pete, the three lads would have felt a real interest in exploring the cavern, now that they had grown accustomed to their surroundings. So far as they had been able to make out, the tunnel they had been treading was partially the work of human hands and partially the work of Nature. The great rift in which lay the accumulation of human remains was evidently the result of some volcanic upheaval. The path, however, was so graded and formed that there seemed no reason to doubt that it had, at one time, been made by the ancient mesa dwellers. "Seems to me we ought to find out what that roaring sound means before we go any farther," suggested Ralph suddenly. "That's a fine Irish bull," laughed Jack. "How are we going to find what it is unless we do go farther?" "That's so," agreed Ralph, somewhat abashed. "Come on, then." A few paces more brought them to an abrupt turn in the path, as they could feel by their constant touching of the inner wall. "Better strike another match," said Walt. "Yes; here goes," agreed Jack. Both boys shouted, to make themselves heard above the now thunderous roaring of the strange noise. A shout of surprise that rose even above the mysterious roaring, followed the striking of the match. Beyond the turn the path took a steep drop downward, and beyond that--the boys could hardly believe their eyes as they gazed--was the glint of rushing water. "The subterranean river!" was the amazed cry that broke from the lips of all three. CHAPTER X. A NEW MEXICAN STYX. "The subterranean river!" The words echoed back weirdly from the vault-like chamber into which they had now penetrated, and at the bottom of which the stream, upon which the light of the match had glistened, flowed rapidly. Within this spacious place the noise was not nearly so loud as it had been when confined in the narrow tunnel, which, in fact, acted much as a speaking-tube would have done. "It can't be!" gasped Ralph, unwilling to believe his own eyes. "But it is," cried Jack, as, all thoughts of their predicament forgotten in this strange discovery, they made lavish use of their matches on gaining the edge of the stream. The river was about twenty feet in width, and they speedily saw that the roaring sound they had heard during their progress through the tunnel was produced by a waterfall some distance above, over which the river plunged into a sort of basin at their feet. But this was not the most astonishing thing they found in that first brief but comprehensive inspection. Affixed to the rocky wall at one side of the chamber was a large, bronze lamp. An eager overhauling of the utensil showed it to be filled with oil, and apparently it was not so very long since it had been lighted.