Hastily applying a match, Jack soon had the rocky chamber lighted, and they could now survey the place into which they had blundered, at their ease. In size it was about the same dimensions as the Council Hall of the mesa, which lay, they knew not how many feet, above them. The river roared down along one side of it, forming a deep, turbid pool just beneath the waterfall, by which it entered the place. To their astonishment, the boys now spied in one corner of the chamber several empty boxes piled up. Remains of excelsior and sacking were within them, and they bore the stencilled marks, "Agricultural Machinery, With Care." Instantly what Pete had related to him concerning the conversation of the men accompanying Black Ramon flashed into Jack's mind. Could it be possible that they had stumbled upon the place utilized by the gun-runners to convey their ammunition across the border? At this instant, there came a shout from Ralph, who had been peering about the place. "A boat!" "A what?" The incredulous cry burst from both Jack and Walt. "It is a kind of a boat, anyhow. Come here, and look for yourselves." Ralph was bending over the rocky marge of the subterranean river at a part of the chamber farthest removed from the waterfall. The water here flowed comparatively slowly, most of its force having been expended in the pool beneath the fall. Sure enough, Ralph had been right. Moored to the bank by two stout ropes attached to iron bars driven into the rock, was a boat--if such a name can be given to the flat-bottomed, floating appliance, upon which the thunderstruck boys gazed. The boat, or rather float, was about twenty feet in length and some five feet in beam. It was not unlike, in fact, one of those shallow craft used by duck hunters, only it was square at each end. Evidently it would hold a considerable quantity of freight. More excelsior and burlap litter in the bottom of it showed that whatever had been the contents of the boxes, it had apparently been used to transport them. "Boys, we've tumbled over the discovery of the age!" exclaimed Jack, in what was for him, a strangely excited voice. The others were not less moved. Their eyes were round and their jaws dropped in incredulous wonderment, as they gazed before them. "Will somebody please pinch me?" It was Ralph who spoke, turning a countenance solemn and startled upon his comrades. "No need to do that, Ralph. You're wide-awake; make no mistake about that." "But--but I don't understand," began Walt in a puzzled tone. "What is this place, what----" "What is it?" echoed Jack. "It's the gun-runners' underground railroad. Can't you see it? This river, so the old Indian legend says, emerges across the border. In some way these Mexicans heard of it, and learned the secret of the hollow altar. No wonder the government has not been able to find out how the rebels got their arms across the border." "Well, what are we going to do, now we've found it?"