Doctor Dan’s knowledge of the country came in play here, for he recognized the place and led the way to their old trail up the mountain, which began about half a mile further along. It seemed useless to spend any more time then looking for Clara Eglinton, so they continued on to the camp, where Doctor Dan cooked a splendid breakfast, having shot an antelope just before they turned off from the plain. The remainder of the day passed without any notable adventure. In the afternoon the rubber boat was launched again and Dick and Charley pulled across the lake to the hut, leaving Doctor Dan to go on an exploring expedition along the shore on his own account. The hut was still deserted and there was no sign that Mudd and his companions had returned. The boys on the way back pulled around to the place where the singular boiling of the water had occurred. All was placid enough now, but just as they were turning away the boiling began again. 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. All hands ran down to the shore and looked off upon the water, but not a thing could they see of old P. D. CHAPTER XI. THE LETTER ON THE TABLE. “He’s around here somewhere,” said Dick. “There’s no mistaking that melodious voice, but where?” Suddenly another strange sound broke upon the air; half scream, half roar, and then a tremendous splash was heard over in the direction of the next cove.