ut suddenly the old man noticed that Quade was not listening. His head was tilted to one side as if he was straining to hear something else. Guinness was held silent for a moment by the puzzled look on the other's face and the strange way he was acting.
"Do you hear it?" Quade asked at last; and without waiting for an answer, he knelt down and put his ear to the ground. When he rose his face was savage, and he cursed under his breath.
"Why, it's a humming!" muttered Professor Guinness. "And it's getting louder!"
"It sounds like another borer!" ventured Sue.
The humming grew in volume. Then, from the ceiling, a rock dropped. They were looking at the cavern roof and saw it start, but they did not hear it strike, for the ever-growing humming echoed loudly through the cavern. They saw another rock fall; and another.
"For God's sake, what is it?" cried Guinness.
Quade looked at him and slowly drew out his automatic.
"Another earth-borer, I think," he answered. "And I rather expect it contains your young friend Mr. Holmes. Yes—coming to rescue you."