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-gla.s.s 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 acc.u.mulation 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 sudden shock of his plight.

"Or maybe the impact of the projectile stunned him!" Tom surmised.

Bud began groping his way upward just as Tom came alongside of him. Tom grabbed him as best he could, hooking onto his belt. At the same time, the young inventor inhaled deeply, yanked out Bud's useless mouthpiece, and inserted his own in its place.

Bud's eyes glowed with grat.i.tude.

"We'll have to get topside fast," Tom thought, "even though it means risking the bends."

He stroked upward and they shot toward the surface. Bud a.s.sisted to some extent, partly revived by the gulp of air.

As they rose, fathom by fathom, their progress seemed to grow maddeningly slower. Tom had to let air bubbles escape constantly from his mouth. As the pressure decreased, due to the lessening depth of the water, the air in his lungs expanded and he was forced to breathe out.

Tom noticed with dismay that Bud was not responding very well, his feeble strokes were jerky and uncoordinated. "Must've lost pressure too fast when his tank was. .h.i.t," Tom realized.

The water was growing greener and brighter now as they neared the sunshine. The _Sea Hound_'s shadowy outline loomed just above. With a last desperate burst of strength, Tom lunged upward and they broke water.

"H-h-help!" Tom gasped.

There was no need for the cry. Hank and his crew, on the seacopter's forward deck, had already grasped the situation. Strong arms reached out and hauled the two boys aboard.

Both of them were shivering and writhing in pain, only half conscious.

"They have the bends!" Arv Hanson cried in alarm. "Signal the _Sky Queen_ to drop a sling!"

The boys' masks were ripped off. Within moments, Bud had been tightly secured to the sling, which was reeled back up into the plane. Tom followed in a few minutes. Doc Simpson took charge of the patients immediately. After a quick examination, he had the boys placed in a small decompression chamber in the _Sky Queen_'s sick bay.

"How are they?" Hank asked anxiously as he peered through the window of the chamber. The medic had given Bud a sedative and he was already fast asleep. Tom remained awake.

"Aside from the pain, not in too bad shape," Doc Simpson replied.

It turned out that Tom's case was not so serious, but Bud had to stay in bed. With Tom, it was only a matter of decompression and he soon was up and about.

Chow, in a chef's cap, with an ap.r.o.n around his paunchy stomach, had come stomping in hastily from the galley. "Pore lil ole boys," he fussed. "Brand my snorkel, I never should've let you young'uns go pokin'

around down below there without me around to keep an eye on things!"

Tom slapped the loyal old Texan on the back. "If you want a dive, come along."

"You're goin' back down?" Chow asked.

"In the seacopter," Tom replied. "To find out, if possible, who fired that projectile at us."

"Then count me in!" Chow declared, stripping off his ap.r.o.n. "I just hope I get my hands on them sneakin' polecats!"

Slim Davis would pilot the _Sky Queen_ back to Shopton at once, because of Bud. Tom and Chow, meanwhile, would join Hank and his crew aboard the _Sea Hound_.

Ten minutes later the sleek seacopter, its searchlight off to avoid detection, was plummeting downward through water that changed before their eyes from greenish blue to a deep-gray gloom. Iridescent fish darted past the cabin window.

"Think the enemy sub was searching for our Jupiter prober?" Hank asked.

"It must have been," Tom reasoned.

Hank frowned. "Which means they must have figured out the missile's position as fast as our side did."

"And they'll play rough to stop us from finding it," Arv added forebodingly.

Within moments, the group cl.u.s.tered in the pilot's cabin felt a gentle b.u.mp as the _Sea Hound_ settled on the submerged plateau. Tom relaxed at the controls but kept the rotors going so the craft would remain submerged. Meanwhile, the sonarman was probing the surrounding waters.

"Any pings?" Tom asked.

The man shook his head without taking his eyes from the sonarscope.

"Nothing yet."