dashed forward. "It may explode!"

Then, as Dick rushed up with his lantern, they saw trailing over the
floor of the tunnel, and on the same side of the stream as themselves,
a thin white fuse, like a sinister snake. It was this burning fuse
which caused the smoke.

It was the work of but an instant for Dick to step on it, and
extinguish the smouldering spark, while it yet had some distance to
travel before the fuse lost itself in a mass of rocks.

"Whew! That was a close call!" exclaimed. Bud, when the fuse was
entirely out.

"Let's see where it leads to," suggested Snake.

They followed it up, and discovered a hidden mine of explosives, tamped
down into a hole that had been drilled in the rocky floor. Iron bars,
hammers and other mining implements showed that the perpetrators of the
dastardly deed had evidently fled in a hurry.

"They were going to blow up the tunnel!" cried Nort.

"And when that collapsed it would mean the end of Flume Valley," spoke
Bud soberly.

"We never could have opened the tunnel again, with all these strange,
branching streams playing around inside."

"But we reached here just in time!" declared Old Billee. "Now let's
get t' th' bottom of this. We know there's a main stream, an' two
branching streams. One of th' branching streams is controlled by th'
water gate with th' copper handle."

"And there must be another gate here, or else Del Pinzo and his crowd
couldn't have shut off the water as they did before they ran away,"
went on Bud. "There must be a whole maze of water-courses in this old
tunnel. Probably the Aztecs dug 'em to save their gold and other
valuables. But I'd like to know what that roaring is?" and as Bud and
the others listened they could hear a subdued murmur, a rumbling and
roaring sound, that seemed to shake the whole tunnel near where they
stood.

"Maybe this leads to it," suggested Dick, as he walked along and
suddenly flashed his lantern across another opening--a natural stairway
leading down into black depths.

"Let's try it," said Bud.

Down it they went, one at a time, carrying their lanterns. And as they
advanced, descending until they came to a level passage, the murmur and
roaring became louder.

"Would you look at that!" suddenly cried. Bud, in an awe-stricken
voice, as he came to a stop and pointed ahead.

And then, as the others gathered about him and looked, they saw a
wondrous sight.

They had entered a cavern, similar to the