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?"

Walt, the practical, propounded the query, as they stood there,
half-stunned by the rapidity with which unheard-of events had happened
within the last half-hour.

"Why, I--upon my word, I don't know," laughed Jack, brou