At last Frank bestirred himself.
"I'm still thinking of that gust of fresh air we felt farther back in the tunnel!" he said.
"There is fresh air coming in somewhere. The air in here isn't getting foul."
"Let's go back and explore the tunnel again. We might find an opening of some kind."
"It won't be big enough for us to get through," predicted Joe, gloomily.
"Well, we'll go and see, anyway."
The boys turned back. Frank took the lead again and they moved on. The flashlight cast its bright circle of illumination on the dank rock walls of their prison as Frank explored every inch of the sides of the tunnel. For a while their scrutiny met with no reward. The tunnel was unbroken by crevice or cranny.
"We must have passed the place by now," said Joe.
"I don't think so. We'll keep on trying."
At last Frank gave an exclamation of satisfaction. He had felt a sudden rush of cold air against his face. It seemed to come from above and he stopped, flashing the light hither and thither.