"Signal guns!" exclaimed Andy. "Some vessel is in distress."
"No, that's thunder!" said Frank. "There's a storm coming up. But we won't know it—in here."
"I hope our boat is safe, and that the Gull is well anchored," went on the younger lad anxiously.
"As if that mattered," thought Frank, but he did not say so. He began to think they would never have any further use for their craft. He choked back the dreadful fear that seemed to take possession of him.
Once more came a terrific clap of thunder, and it seemed to shake the very island to its center.
"It's a fierce one," murmured Andy.
In quick succession came a number of awful reports. The earthy wall to which they were clinging seemed to tremble. The water gurgled below them, rising higher and higher.
"I wonder—" began Andy, after a terrific clap, but his words were silenced in the thunderous vibration that followed. It was the hardest clap yet, and the boys felt a tingling, numbing sensation in their fingers.
"That struck near here!" yelled Frank.
His face was turned upward toward the roof of the cavern. He felt something falling on his cheeks. It seemed to be particles of dirt. Then he felt a dampness that was not from the waters below him. More particles fell.