“Jack was right!” Walter exclaimed. “This is a cavern, and it looks to be a good-sized one. I wish we had a light.”
“Here’s a lantern,” said Paul, who had lighted another match. “We’ll explore a bit.”
By the greater light of the lantern, which was found near the doors, the boys saw that the cave was indeed a large one, extending well back under the hill. They went in cautiously at first, not knowing what they might find, or what hidden pitfalls might lie in their path.
“Look!” exclaimed Jack, pointing to several boxes lying about. “They must have been doing, or else are getting ready to do, lots of business. Those boxes contain paper and cardboard by the looks and marks on them. And now——”
“Hark!” exclaimed Paul in a whisper. They all listened. From somewhere far back in the cave came a dull, rumbling, vibrating noise, and the ground faintly trembled.
“There it is again!” said Walter—“that strange noise. Now we’ll find out what it is. Come on.”
He started forward, the others following, Paul in the rear with the lantern, for it had a reflector on and gave better light when carried behind the boys.
“Wait a minute!” cautioned Jack. “I don’t seem to hear that noise now. It’s stopped.”
“So it has,” concurred Paul.
They listened intently, then Jack said: