The boys looked at each other in amazement. During all of the previous visits there had never been the slightest sound within the cavern.
"Possibly," said Harry, "it may be running water."
"It doesn't sound like water to me. I will—" but Harry did not have an opportunity to say anything more, as a terrific roar, like a cannon shot, rang out, and the boys were simply petrified.
"What do you think that was?" whispered George. They drew close together, and spoke in whispers.
"This will never do," declared Harry. "If there is anything in this place we might as well know it now as later. Will you join me in the hunt?"
And the boys responded with one assent. "Let us go to the second chamber. Come on, boys." And they bravely stalked down the corridor.
When the chamber was reached a hollow laugh greeted them, followed by two hearty laughs. The Professor and John had entered the opening at the sea end, and hurriedly made their way to the second chamber, where they awaited the coming of the boys.
The boys were intensely relieved, and the Professor was so happy to see the determined and resolute spirit they manifested, that he complimented them highly.
John was no less profuse in his commendations. "I want to say, that men can be brave when they know what they have to fight, and who their enemies are; but it takes the stoutest heart to go forth and defend yourself, or assume the offensive against an unseen and an unknown enemy."