ed and swam toward the apparent direction
of the sound. At that moment a distant wail struck their ears.

The brass ball!

Rick wondered. He had heard no boat noise. The brass ball must be
operating automatically. He hooted for Scotty's attention, then pointed
toward the cave.

Scotty checked his spear gun and motioned for Rick to lead. Rick pushed
his camera in front of him and made his fins move rapidly. There might
be some outward sign when the ball sounded, something that would tell
them a little about its mechanism or purpose.

As the cave came in view he coasted, looking upward. The murky layer
prevented his seeing very far, but there was no one in sight. He let
inertia carry him toward the cave, then swung upright in the water as he
saw that the octopus had moved a little distance from the cave mouth.

Instinctively Rick knew that something was wrong, but it was too late to
get out of harm's way.

A frogman emerged from the cave, spring-type spear gun pointed directly
toward them. The frogman held the brass instrument in his free hand.

Even as Rick hooted a warning, the frogman fired!

His spear lanced through the water directly at Scotty!

CHAPTER XVII

Trapped in Twenty Fathoms

Scotty writhed to one side, and the fact that the frogman had fired from
too great a distance gave him time to dodge. The spear went by, and
Scotty lifted his own gun to return the shot.

Rick, senses suddenly acute, glanced upward again in time to see two
more figures descending through the murky layer. He hooted for danger!

Scotty glanced up, too. Then, instead of firing, he sped forward and
thrust the tip of his spear at the frogman's chest. The frogman lifted
his hands high. Scotty jerked the man's face plate loose, then turned
swiftly and motioned to Rick.

Rick followed, fins driving, as Scotty led the way into deeper water in
the direction of the wreck.

The frogman who had been in the cave was temporarily out of things. His
Scuba was the type that combined the breathing apparatus with the full
face plate. He could clear the face plate of the water Scotty had let
in, but it would take a little time.

Suddenly Scotty shot upward. Rick turned and looked over his shoulder as
he followed. The second two frogmen were in clear water now, and both
had spear guns!

Scotty led the way into the murky layer, then leveled off and swam
horizontally. Rick wondered what kind of evasive action his pal was
planning