it
back to where you found it, Bud, and see what happens."

It was with some feelings of apprehension that the others watched as
Old Billee reached for the copper lever and pulled it toward him, It
operated as easily as it had for Bud.

And almost as quickly as had taken place on the other occasion, there
was that roaring, rumbling sound, a noise as of the blowing of a great
wind, and then the waters began to rush back into the pool.

"Here they come!" yelled Dick, as he stood beside Bud on the rock.

Truly the waters were returning as the hidden gate was closed when
Billee pulled the lever.

Would they go down again?

That was what each one asked himself.

CHAPTER XXIII

THE CONSPIRATORS

Rapidly rushing, foaming, bubbling and boiling, the waters rushed into
the mysterious cavern, until they again filled the pool across which
Bud and Dick had swam to the rescue of Nort on the rock. Now the
situation was reversed. It was Nort who was on the mainland, or shore,
so to speak, and the others who were on the rock.

But it was one of their own choosing, in an endeavor to solve the
mystery, though as Bud and his companions watched the waters creeping
higher and higher up the surface of the rock on which they stood, their
hearts were not altogether easy.

"Suppose it covers the rock?" asked Dick.

"Then we'll have to swim back where Nort is," Bud answered.

"Shucks! You won't have to do nothin' of the sort!" declared Old
Billee stoutly. "She won't come up any farther than it did before!"

And he was right. When the water around the rock lapped the erosion
mark, which had been worn in the hard stone by centuries of the flow of
the fluid, the flood ceased. The roaring, bubbling and seething, like
that which takes place in a canal lock, came to an end, and the water
of the pool became quiet.

"There! What'd I tell you?" cried Old Billee. "I closed th' water
gate, that Bud opened to let th' water out, an' she come back. Now all
we have t' do, so we can walk back, is t' yank this lever again."

"Does it only work two ways?" asked Yellin' Kid, his voice again
softened, as the mystery of the place seemed to cast a shadow over him
and the others.

"Seems to," Bud answered, holding his lantern down close to where the
copper handle entered the rock.

There appeared to be a slot cut in the hard stone--a slot about three
inches wide, and a foot long, in which the copper lever could be moved
backward a