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.
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