into full swing reached their ears.

It was a 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.