“Hear what?”
“That strange, rumbling, trembling noise. Don’t you feel it?”
“Yes! Yes!” cried Belle. “Oh, what is it?”
There was no doubt of the noise. It seemed to fill the whole passage with a dull, rumbling roar, and the ground vibrated and trembled.
“Come on!” cried Cora, resolutely. “It’s just ahead of us. We will solve the mystery now!”
Willing or unwilling, Belle, Bess and Hazel followed their leader. With their electric lights showing the way the girls pressed forward. Suddenly the passage turned, and, making that turn, the girls came upon a strange sight.
Before them was an open door, which gave entrance to a large cave with rocky sides and roof. Vaulted and large the cave was, and from long wires fastened somewhere in the roof hung a number of incandescent lights. In the cave the girls saw several queer machines, and Cora, at least, recognized more than one of them as printing presses. A gasoline engine was throbbing away in one corner, and it was this, Cora decided, which made the rumbling, the throbbing and trembling vibrations.
Hardly realizing what they were doing, the girls walked forward, and, passing through the open door, entered the cave which widened out at the end of the secret passage.
“What—what does it all mean?” asked Bess.
Low as her voice was it seemed to awaken strange echoes in the vaulted cave. And at the sound of it something stirred in one corner. From a pile of boxes something arose—a something that resolved itself into an old man with white hair and a long, white beard. He peered from beneath his bushy white eyebrows, with piercing eyes at the startled girls, and from his throat came a guttural cry.