tremble.

"What have you done?" gasped Dick.

"Search me!" answered Bud in such queer tones that Nort laughed.

And then a strange thing happened. As Nort struck another match he and
the boys on the rock could see the water all about them beginning to
recede. Slowly it flowed at first and then, with a rush, it began
running out of the place as fast as it had run in.

"What's up over there?" called the voice of Old Billee from "shore," so
to speak. "What you fellers doin' with th' water?"

"I just pulled that lever," sang out Bud.

"Then you've done the trick!" said the old cowboy. "You must have
opened some gate, and the water's running away. Better swim over here
while you have the chance. When the water comes back that rock may be
covered!"

But another strange part of their mysterious adventures was that they
did not have to swim back. For the water receded so rapidly that, in a
little while, it was possible to wade from the rock to the stone edge
of the pool where the other members of the party stood. And wade back
to their friends Bud, Dick and Nort did.

"Oh, boy! But we're glad to see you!" cried Old Billee, as he caught
Nort by the hand.

"You let out a mouthful that time!" declared Yellin' Kid, and his voice
nearly split their ear drums, so magnified was it by the echoing,
vaulted roof of the cavern.

"But what all happened?" asked Snake Purdee. "Is there some old
Mexican grain mill under here that has a water-wheel, sluices and
gates?"

"I give it up," answered Bud. "All I know is that I pulled that copper
lever--and it's copper so it won't rust off, I reckon--and the water
began to rush out as fast as it must have come in here."

"It is mighty queer," agreed Old Billee. "Let's go take a look," and
he started to walk across the intervening space between shore and the
great rock--a space in which only a few puddles of water now remained.

"Will it be safe?" asked Bud, who had begun to dress, an example
followed by Dick.

"Why not?" asked Old Billee. "The water can't rise any higher than it
was when you fellows were on the rock. An', according to your tell,
there's room enough for us all t' stand there."

"Yes, it's big enough," agreed Bud. "But suppose we all get there, and
the water begins to come back?"

"We'll turn it loose again with th' lever," answered the old cow
puncher. "But I reckon it can't fill up this pool again until that
lever is shifted hack where it was be