ion, and the boys soon had it
swinging to and fro on the surface of the water which now lay several
feet deep in the main gangway.

"Je-rusalem!" exclaimed George, taking the depth of the water with an
oar, "if the water is four feet deep here, how deep must it be at the
middle of the dip?"

"About forty rods, I should think!" exaggerated Will.

The boys left a large searchlight at the shaft, so situated that it
looked straight down the passage they proposed following, and started
away in the boat. The flashlights illuminated only a small portion of
the underground place, but the boys could see some distance straight
ahead.

Once they ceased rowing to listen, believing that they had heard calls
from the darkness beyond. The sound was not repeated, and they were
about to proceed when a sound which brought all their nervous energy
into full swing reached their ears.

It was the bumping of an oar or paddle against the side of a boat. The
blow echoed through the cavern as sharply as a pistol shot might have
done. There could be no mistake in the cause.

"Now who's in that other boat?"

"Somehow," George grumbled in a whisper, "we always have propositions
like that put up to us! There's always a mystery in every trip we take!
We found one on Lake Superior, and one in the Florida Everglades, and
one at the top of the Rocky mountains and one in the Hudson Bay
wilderness."

"Yes, and we solved them, too!" grinned Will. "And we're going to solve
this one! You remember about my seeing some one sneaking in here just
ahead of us, don't you?"

"Yes," was the answer. "You thought it was that bum detective."

"I think so yet," replied Will.

"If it's the detective," asked George, "why didn't he give the alarm
when he found that the mine was being flooded. He might at least have
done that and saved the company a great deal of expense and trouble."

"Give it up," replied Will. "I might ask you," he went on, "why he was
rowing away into a flooded mine which is supposed to be deserted."

"And I'd have to give you the answer you gave me," George declared.

The boys could now hear the strokes of the oarsman who was in the lead
quite regularly and distinctly. Now and then he turned into
crossheadings and chambers, as if to escape from their surveillance, but
they kept steadily on after him, not taking into account the fact that
they were leaving the light they had set at the shaft far out of view.

"Perhaps we ought to tur