s broken into by a sharp signal
from the submarine:

"Pardon interruption! Hurry back! We are attacked!"

At this, the trio stood rigid.

"Captain Petersen! Captain Petersen!" Larry heard the professor call.
"Speak up! Give details! What has happened?"

But an ominous silence greeted the query.

Another moment they stood there, thoroughly dismayed now. Then came
the professor's swift command:

"Follow me--quickly!"

He was already in motion, retracing his steps as fast as his bulky
suit would permit. But as he rounded the corner of the pyramid, they
saw him pause, stand staring. And as they drew up, they in turn
paused; stood staring, too.

With sinking hearts, they saw that the _Nereid_ was gone.

* * * * *

Stunned by this disaster, they stood facing one another--three lone
human beings, on the bottom of the Atlantic ocean, their sole means of
salvation gone.

Professor Stevens was the first to speak.

"This is unbelievable!" he said. "I cannot credit it. We must have
lost our senses."

"Or our bearings!" added Diane, more hopefully. "Suppose we look
around the other side."

As for Larry, a darker suspicion flashed through his mind. Captain
Petersen! Had he seized his opportunity and led the crew to mutiny, in
the hope of converting the expedition into a treasure hunt? Was that
the reason he had been so willing to remain behind?

He kept his suspicion to himself, however, and accompanied Diane and
her father on a complete circuit of the pyramid; but, as he feared,
there was no sign of the _Nereid_ anywhere. The craft had vanished as
completely as though the ocean floor had opened and swallowed her up.

But no, not as completely as that! For presently the professor, who
had proceeded to the site where they left the craft resting on the
sand, called out excitedly:

"Here--come here! There are tracks! Captain Petersen was right! They
were attacked!"

Hurrying to the scene, they saw before them the plain evidences of a
struggle. The ocean bottom was scuffed and stamped, as though by many
feet, and a clear trail showed where the craft had finally been
dragged away.

Obviously there was but one thing to do and they did it. After a brief
conference, they turned and followed the trail.

* * * * *

It led off over the plateau a quarter mile or more, in an eastward
direction, terminating at length beside one of the smaller
pyramids--and