Hardly had he spoken than there rang throughout the cave a dull, booming sound. It seemed to shake the ground.
"He's exploded something!" cried Frank, coming to a halt. He flashed his electric torch around, but could see nothing. He and his brother were in a low, rock-roofed passage.
"It sounded like something falling," was Andy's opinion. "Let's go forward and see what it was."
They had not gone forward more than a dozen steps before they were halted by the sound of a voice—the voice of the mysterious man.
"Maybe you'll take a warning next time!" he sneered. "I think you've followed me once too often. This is the end."
They could hear him hastening away. Then came silence.
"What did he mean?" asked Andy.
"I don't know," replied his brother. "Let's look."
Andy was in the lead. Slowly he advanced, flashing his electric light.
Then he came to a halt.
"What's the matter?" asked Frank.