The Celt was nothing if not curious.
It looked like a strange formation of some aquatic growth, and unthinkingly Barney jabbed the point of his knife into it.
The result was thrilling.
The round body instantly contracted and then rebounded, throwing Barney backward with great force.
And then out of the darkness of a cavern passage came a great flat head with horrid jaws.
It was a species of sea serpent. The huge coils were thrashing the water of the cavern furiously, and the divers stood for a moment paralyzed with terror.
The sea serpent was undoubtedly the habitue of the ocean cavern. He seemed also disposed to resent this invasion upon his chosen territory.
The situation was critical.
Frank saw at once that quick action must be made, or serious consequences would be the result.
The young inventor, therefore, at once signaled the others to follow him, and began a retreat for the Dart.