s seat.

"Got a job for me, skipper?" asked Doc Simpson, Swift Enterprises' young
medic.

"Yes. Help the boys, if you like, rig the undersea elevator, and then
assemble a tractorized air dome," Tom suggested.

"Will do," Doc promised.

A ladder was dropped. Tom and Bud excitedly descended to the _Sea
Hound_. The search for the lost missile was about to begin!

Once the boys were aboard, the seacopter submerged and dived quickly to
the ocean floor. Tom and Bud each climbed into a Fat Man suit and went
out through the air lock. The suits, shaped like huge steel eggs with a
quartz-glass view plate for the operator seated within, had mechanical
arms and legs.

The boys waddled about, the built-in searchlights of their suits
piercing the murky gloom. They saw nothing but the deep accumulation of
silt on the ocean bottom, which made the going difficult.

"This is too slow," Tom called over his sonarphone. "Let's try the air
dome."

The dome was a huge underwater bubble of air, created by a repelatron
device which actually pushed the ocean water away. The air supply inside
was kept pure by one of Tom's osmotic air conditioners which made use of
the oxygen dissolved in the water.

The air bubble, however, even with its jet-propelled platform, also
proved inadequate for the research job. Its caterpillar treads
repeatedly bogged down in the silt.

"Maybe the seacopter itself is our best bet," Bud suggested.

"Worth a try," Tom urged.

But the _Sea Hound_, too, had a serious drawback. Even with its powerful
search beam sweeping the ocean floor as it prowled along, the explorers
found their vision too limited.

Finally Tom said, "Bud, we could skin-dive at this depth."

"Let's give it a whirl," Bud urged.

The seacopter surfaced again, while the boys donned flippers, masks,
and air lungs. Then they dropped over the side and made their way slowly
downward into the gray-green depths, accustoming themselves gradually to
the increased pressure.

"A lot more freedom of action," Tom thought. "If only we didn't have to
communicate by signals!"

There was a sudden _swoosh_ somewhere on his right. A projectile, Tom
realized! Turning, his eyes widened in horror as he saw an uprush of
bubbles.

Bud's air tank had been hit!

CHAPTER III

INVISIBLE SUB

Without wasting a moment, Tom lunged through the water toward his
stricken friend. Bud was floundering and thrashing about weakly. He
seemed dazed by the su