They were almost at the entrance of the second cave when they heard a faint sound.
Joe wheeled about.
"What was that?"
They listened. The sound was repeated. It was like a distant cry.
"Somebody calling!" declared Biff excitedly.
"It must be Frank!"
The boys stood quite still and listened for a repetition of the call. It came again, muffled and far away, but unmistakably a human voice.
With one accord, they turned and ran back into the cave.
"It's Frank!"
They hurried across the treacherous floor in the direction of the sound. It was clearer now.