A rumbling sound, somewhat as if a heavy train were passing overhead, filled the cavern. It shook violently and the waters of the lake became wildly agitated. The monsters at once left their feast and sank into the lake, leaving the mangled body of their dead mate floating on the surface.
The rumbling grew louder and the cavern shook till the lake was lashed into little wavelets.
“It is the voice of the Trembling Mountain,” said the professor solemnly; “somewhere the mighty forces of nature’s forges are at work.”
CHAPTER XXIII.
THE HEART OF THE MYSTERY.
“Sounds more to me as if Mrs. Nature had a tummy ache,” said the unromantic Coyote Pete.
But nobody laughed at this remark. The sounds were too awe-inspiring. Suddenly they ceased as abruptly as they had begun, the rumblings dying out like a sharp clap of thunder.
“Is there any danger?” inquired Jack.