signal from Bud!" Zimby exclaimed.

Tom nodded grimly. "He's spotted trouble--probably an enemy sub."
Silence settled over the cabin as Tom reached out to switch on the
antisonar circuits.

At that same instant a red light flashed on the control panel. "The
Damonscope!" Tom cried out. "We may be over the Jupiter prober!"

Cutting off the steering jets, Tom gave a brief flick on the reverse
jets to halt the craft. Then he turned over the controls to Zimby and
began stripping down to don a hydrolung suit.

"Gallopin' guppies! What're you aimin' to do?" Chow exploded.

[Illustration]

"Go out and look for that missile," Tom said calmly. "It's what we came
for."

"Are you loco, boss? What about that sub Bud just spotted? Mebbe it's
Mirov's bunch!"

Tom refused to be dissuaded. After swallowing a space-plant pill, he
armed himself with an underwater flashlight.

"Think it's safe to show that light, skipper?" a crewman asked uneasily.

"If the enemy spots it, I'm hoping they'll think it's coming from a
school of lantern fish or sea anglers," Tom explained. He picked up a
three-pronged digging fork with his other hand and went out through the
air lock.

Tom glided back to the spot which the _Sea Hound_ had just passed over
and began digging into the silt. Presently he felt the fork strike
something hard.

"An obstruction!" Tom thought excitedly.

He probed deeper. Bit by bit, a smoothly contoured and still-shiny metal
surface became visible. "I've found it!" Tom's eyes flashed in triumph,
his heart pounding.

There was no doubt he had uncovered the nose cone of the missile which
had re-entered the earth's atmosphere tailfirst!

Meanwhile, Bud, keeping watch on the enemy submarine, had seen a shadowy
figure glide from its air lock and head in Tom's direction. Bud donned a
hydrolung and followed.

"What's that he's carrying?" Bud wondered.

Suddenly the answer came to him--a self-propelled underwater grenade!
Horrified, Bud jetted forward, tackling the diver at full speed.

A split second too late! The grenade went streaking straight toward Tom
Swift!

CHAPTER XX

A LUCKY BLAST

Tom's earphones caught the hiss of the approaching grenade. Instantly
his eyes darted to the sonarscope on his wrist.

A tiny blip of light was moving on the screen!

Tom whirled about, then gunned his ion drive. He pushed out of the path
of the grenade, which nevertheless grazed him as it streaked past.

Seconds