The boys watched it until it ceased, the time being exactly ten minutes.
After it was over they waited around for half an hour more, but the phenomenon was not repeated.
“We must come over and spend the day here soon,” declared Dick. “My theory is that if an underground lake really exists that is where old P. D. and the other monster have their holdout and we must contrive some way to get down into it. Maybe the underground passage I went through connects with your cavern, Charley. What do you say to following it up and trying to find out?”
“That’s the idea, exactly,” said Charley. “We know from your discovery that there is a small underground lake, so what’s to hinder there being a big one? We’ll take that in to-morrow and the event will prove that I am right.”
On the way back to camp Charley thought that he caught sight of the head of old P. D. lifted for a single instant above the water, but Dick did not see it and Charley could not feel quite sure.
When they got back to camp they found that Doctor Dan had already returned and had supper ready.
“I’ve got great news to tell you, boys,” he exclaimed. “I’ve seen old P. D. again and this time on the land. What do you say to that?”
“Hooray!” cried Dick. “Here’s another discovery of the habits of old P. D. It gives me hope that we may succeed in capturing him yet.”
“There’s a chance for us,” said Doctor Dan, “and I can show you just how it can be done if you will follow me around the lake shore to a place where I was to-day.”
He had scarcely made the remark when the same old bellow was heard off on the lake.