they probed about with no sound trace of
the "invisible" jetmarine.

But presently Arv snapped his fingers to catch Tom's attention. "Got
her, skipper!"

Tom took over the hydrophones. Sure enough, his ears could make out the
faint hum of the jetmarine's atomic turbines. Tom directed Hank toward
the sound, then ordered him to switch on the _Sea Hound_'s powerful
search beam.

The light cut a path of radiance through the murky dark-green waters.
Dead ahead, the jetmarine could be seen gliding across their field of
view.

"Your system blinded our sonar okay, skipper," Hank commented, "but this
proves she could still be spotted by enemy listening devices."

Tom refused to be discouraged. He ordered Hank to return to base and
wait for Bud. Meanwhile, the young inventor applied himself to the
problem of how to mask the sub's noise.

"How about it, pal?" Bud asked, when he reported aboard the seacopter a
while later.

Tom explained the results of the test and the need for an added
safeguard against hydrophone detection. "Think I see a simple way out,
though," he added with a pleased chuckle.

"Natch! With a brain like yours, it's a cinch," Bud quipped. "Explain,
professor."

"Well, we can never do away with the noise of a sub's propulsion
machinery," Tom began. "That goes without saying. So we'll have to
camouflage it--lose it in the underwater jungle noises, so to speak."

Bud scratched his head. "How do we do that?"

"By amplifying the natural undersea sounds all about it," Tom explained.
"Fish and all forms of underwater life make a background noise over the
hydrophones, you know."

As Bud nodded, Tom went on, "So we simply step up the volume till the
sub's own noise gets drowned out or 'wasted' in all the racket."

This could be done, he concluded, with fairly simple amplifying
equipment. Bud, Hank, and Arv were jubilant at the idea.

"Nice going," Bud said. "How soon can we give it a try?"

"Soon as I can rig up the amplifier," Tom promised.

In less than two hours they were ready to submerge again. Zimby Cox
joined the crew. Bud suggested taking along hydrolungs in case of any
need for tinkering with the transducers or amplifying equipment.

This time, the jetmarine scored perfectly on the test, successfully
eluding all the _Sea Hound_'s efforts to detect it. Tom returned happily
to base, feeling that the antidetection problem was now solved. The
jetmarine, however, failed to appear.

"That's fun