w and then to make
sure they were alone in the water. The little octopus was in his usual
position on the ledge.

Scotty, spear gun extended, swam right into the cave. Rick followed,
holding the camera tightly to his chest to keep it from scraping on the
coral. Scotty had his flashlight going, so Rick didn't bother with his
own.

The cave was just about large enough for both of them. It was a typical
coral formation, not much different from the reef outside, except that
the brass ball was in the center of the rough floor.

The boys examined the cave thoroughly and saw nothing of interest. Rick
pushed at Scotty's shoulder and swam out again. Scotty followed. The
octopus watched them go.

The wreck of the _Maiden Hand_ was just as they had left it, and the
grouper was back in his comfortable cabin. He departed at high speed as
the boys appeared. They had agreed to start work aft of the captain's
cabin, and the wrecking bars were carried under their tank harnesses for
the purpose. Both were convinced that there was nothing more to be found
in the cabin, although the possibility remained that false boards in the
floor or walls might conceal the statue.

Rick tied his camera to a projection, then took his wrecking bar and
looked for a place to start. Scotty pointed to a place where there were
boards aft of the cabin they had already uncovered, and they started to
work.

By the time they had pried off the first few boards it was time to
surface. They went topside and changed tanks, then rested for half an
hour. There was no sign of activity at the frogmen's house, nor could
they see Tony at work on his midden, since the location was hidden by
palms.

Rick said thoughtfully, "The brass ball might be some kind of signaling
device."

"What kind of signals?"

He didn't know the answer to that. "Anyway, since it's underwater, if it
sends out anything it must be sound impulses. Otherwise we wouldn't hear
it wail. And what good is sound if not for signals?" added Rick.

"Sonar," Scotty reminded.

The boys were familiar with sonar because of the Spindrift work on the
Submobile. Very high frequency sound impulses were sent out, and the
echoes were timed or used in other ways. It was the way in which bottom
tracings were made by surface craft, and the way in which Navy ships
detected submarines. It could be used for locating schools of fish.

"It could be sonar of some kind," Rick agreed. "But what good would it
do any