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."