d
the mainland. Fathometer soundings showed it was on a steep upward slope
of the continental shelf.

Presently a foaming gush of bubbles showed that the sub ahead was
blowing its tanks. The jetmarine followed as it surfaced and Bud hastily
manned the periscope.

"What're they up to?" Mel asked tensely.

"Don't know yet, but the hatch is opening," Bud reported. Suddenly he
gave an excited gasp. "Jumpin' jets! They're sending out a couple of
frogmen!"

Bud's companions were electrified by the news.

"Spies!" Zimby exclaimed.

"What do we do now?" piped up Mack Avery, the third man in Bud's crew.
"Hadn't we better radio the Coast Guard and the FBI?"

Bud wrenched away from the eyepiece. "I have another idea! Any of you
fellows game to go with me and capture those spies?"

All three of his companions volunteered eagerly. Bud chose Mel Flagler,
then took another sight through the periscope.

"The sub's submerging again," he reported. "That'll give us a clear
field. Zimby, you and Mack keep an eye on that baby while we're gone,
and be plenty careful she doesn't spot you!"

"Roger! And take this roll of wire to tie up your prisoners."

Hastily Bud and Mel changed into swimming trunks and donned hydrolungs.
They went out through the air lock, plunged into the bracing salt water,
and switched on their ion-drive units.

"Can you see 'em?" Mel asked over his mike.

"Not yet. Let's speed up before we lose 'em completely!"

Both pushed their ion drives to capacity, scanning the water ahead in
all directions.

"There they are!" Bud exclaimed presently. He pointed to two tiny
figures, barely visible in the distance.

"Wow! They're sure not wasting any time!" Mel muttered. "Let's step on
it, Bud! They'll be ashore in a minute!"

A darting school of sea bass screened the figures briefly from view. As
the fish flickered past, Mel and Bud saw the frogmen breast-stroke up
toward the surface and break water.

Bud and Mel followed. Ahead lay a barren stretch of beach, humped with
sand dunes. It was skirted beyond by a thick fringe of trees.

"They certainly picked a perfect spot for a sneak landing!" Bud thought.
The beach seemed totally deserted, with no sign of human habitation.

By this time, the frogmen were scrambling ashore. Within moments, Bud
and Mel were on their heels. The raiders whirled in dismay as they
caught the sound of footsteps rushing up behind them through the sand.

Bud and Mel hurled themselv