_Sea Hound_," Tom said, pointing north-northwest.
"Submerge as we go!"

Bud circled his thumb and forefinger, then adjusted his mask, and the
two boys plunged back in. On a sloping downward course, they sped along
like undersea rockets, their ion jets functioning perfectly. Minutes
later, they sighted the seacopter.

Hank waved to them through the cabin window as they glided past. The air
lock opened speedily and the two boys entered. Both heaved sighs of
relief when they were safely inside.

"Somethin' wrong?" Chow asked, sniffing trouble.

"A strange submarine," Tom reported. "Brungarian more than likely. It
may be heading this way if they've tracked us."

"A sub?" Hank was startled. "We've picked up nothing on sonar!"

"Check again," Tom ordered.

The sonarman bent to his scope and Hank listened intently over the
hydrophones. Neither could detect any sign of another craft.

"Probably the same one that fired on us the last time," Tom said grimly.
"We'd better clear out before they take another pot shot at us."

Hank sent the _Sea Hound_ zooming toward the surface while the boys
changed quickly into slacks and T shirts. Then Tom took over the
controls for the flight home.

"Brand my vitamin vittles! Are we just goin' to turn tail an' run every
time them varmints come skulkin' around?" Chow fumed as the seacopter
arrowed northward.

"Not if I can help it," Tom vowed. "But first I must figure out a way to
make our own craft invisible, so to speak. It's the only way to protect
our American crews, Chow, if we hope to do any secret digging for that
lost missile."

"Want another suggestion, skipper?" Bud put in. "This one is about the
hydrolung."

"Sure. Speak up."

"How about putting some sort of communications system into our hydrolung
gear? If I hadn't been close enough to grab you when I spotted that
sub, it might have been curtains, pal!"

"You're right," Tom agreed. "I'll get to work on it."

It was sunset when Fearing Island came into sight. The boys flew a
Pigeon Special back to Enterprises, where Tom phoned a full report on
the mystery sub to the Navy Department. Then the two chums drove to the
Swift home for a late supper.

Phyl Newton was visiting Sandy that evening, but the girls displayed a
marked coolness toward Tom and Bud. Instead of engaging in conversation,
they retired to Sandy's room upstairs to play records, while Mrs. Swift
served the boys a warmed-up but tasty meal of roast beef a