ny. The test was over at four-fifteen," Tom murmured.

"Maybe Bud surfaced out at sea somewhere," Arv Hanson suggested.

Repeated radio calls brought no response. Tom, now seriously worried,
took the seacopter down again for another search, hoping that Bud would
have switched off the antidetection gear by this time. But neither
sonarscope nor listening devices revealed the slightest clue.

Tom, Hank, and Arv exchanged fearful glances. Had the jetmarine
foundered on the ocean bottom--perhaps fouled somehow by Tom's new
invention? Or had Bud and his crew fallen victim to the enemy?

CHAPTER XIII

ENEMY FROGMEN

At the end of the test period, Bud had prepared to bring the jetmarine
to the surface. But just as he was about to blow the ballast tanks, Mel
Flagler sang out a warning from the sonarscope.

"Whoa! Hold it, skipper! I think we have company on the starboard beam!"

Bud jerked his head around in surprise. "You mean the _Sea Hound_?"

"No, she surfaced," Mel reported. "Can't make this out yet, but it could
be another sub."

Bud turned the controls over to Zimby Cox. Then he rushed to the scope
and examined the blip. "Seems to be moving away from us on a westerly
course. It's about two miles from here."

He donned the hydrophone earset and listened. "It's no seacopter, nor a
jetmarine either," he announced presently.

"A Navy sub, maybe?" suggested Zimby.

Bud shrugged. "Let's find out." He ordered a change of course, hard to
the right, and gunned the jets to bring the jetmarine directly on the
mystery object's trail.

"It's a sub, all right," he said a short time later, listening again
over the hydrophones.

"Pretty close to Fearing Island, isn't it?" put in Mel Flagler. "That's
a government-restricted area."

Bud nodded grimly. "But staying just out of sonar range from the base."

The jetmarine closed steadily on its quarry. In a few minutes they were
able to make it out dimly through the cabin window, dead ahead.

"That's sure no U.S. Navy sub that I know of," Bud said. "Probably an
enemy snooper."

"What if they spot us?" Zimby asked.

Bud chuckled. "That's the beauty of it, pal! Don't forget. With this new
antidetection gear we're invisible to them. At least as long as they
don't run into us or we into them," he added.

"Or unless they have superdetection equipment we don't know about,"
cautioned Mel Flagler.

As they talked, the unidentified submarine was bearing steadily towar