Everything about it--the smoothly-cut rock walls, roof and floor--the absence of débris and litter, the shape of the cavern, indicated that it had once formed a subterranean channel for the river, or for a portion of it. Perhaps even now it formed an overflow for the waters when in flood. And as this dawned upon me, a new hope rose in my breast. Perhaps, possibly, the cavern might connect with the open air. By following it, we might come out above the falls. There were one hundred chances to one against it; we might find it ended in narrow cracks or fissures; it might lead to a perpendicular shaft; it might bring us to the verge of an underground pool or stream. But the one chance was worth trying, and, having breakfasted upon the last of our provisions, we started on our exploration of the tunnel. * * * * * FOR an hour or more we crept forward. The tunnel remained the same size; it led gradually upward, and my hopes that it would lead us to freedom increased less crawling, we saw a faint light ahead, I felt sure we would soon be safely in the outer air. Brighter and brighter the light became, and we hurried forward. The light came from above and illuminated a mass of fallen rock that half-filled the tunnel before us. As we reached the débris and I started to scramble over the stones, the air suddenly vibrated with a strange whirring sound that seemed to issue from every side. For an instant I hesitated, listening, yet vaguely conscious that I had heard that same sound before. The next instant I leaped back with a startled cry. From a crevice among the rocks, a great flat, arrow-shaped head had darted out, had struck viciously at me, and had missed me by the fraction of an inch. Now I knew. The place was a den of immense rattlesnakes! * * * * * [Illustration: The next instant I looked back with a startled cry] * * * * * The chief fairly shook with terror, his eyes rolled and he glanced furtively to right and left, seeking some spot where his naked skin would be out of reach of the angry serpents that now were wriggling, coiling, swarming among the rocks about us. Again the prophecy of Kukulcan had been borne out; here indeed was the tunnel of the serpents!