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 gratitude.
 
"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 assisted 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 hit," 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 apron 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 apron. "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 clustered in the pilot's cabin felt a gentle bump 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."
 
Hank Sterling donned a hydrophone headset and listened intently. The silence deepened in the Sea Hound's cabin. Suddenly Hank stiffened and the sonarman cried out:
 
"A blip, skipper! At two o'clock!"
 
It was moving rapidly on the scope—something streaking toward their starboard beam!
 
"Good night! It's another missile!" Tom gasped.
 
He darted back to the controls and gunned the reverse jets just in time! The missile flashed across their bow.
 
"Great bellowin' longhorns!" Chow gasped weakly. His leathery face had gone pale under its tan. "The yellow-livered drygulchers!"
 
"I don't get it," Arv Hanson spoke up. "If they're in firing range, we should have detected them, shouldn't we?"
 
Tom nodded grimly. "Whoever our enemies are, they must have perfected a way to make themselves invisible to underwater detection.
 
"And we'll have to do the same!" he vowed inwardly. Aloud, Tom said, "I hate to run from those sneaks, but if we stick around, we'll be asking for trouble."
 
Slowing the rotors to permit the craft to rise, Tom guided the Sea Hound back to the surface. Then he reversed blade pitch for air flight and gunned the atomic turbines. The seacopter rose steeply above the billowing South Atlantic.
 
Tom radioed a terse report of their experience to the task-force commander and in turn was told that none of the naval craft had either sighted or picked up any sign of a strange sub.
 
As they streaked homeward, Chow was still fuming. "Why don't we post a dummy sub there to scare off the varmints?"
 
"I'll pass the idea along to the Navy," Tom said with a grin.
 
Night had fallen when the searchers arrived back at Fearing Island. Tom cleared with the tower and landed, then went by jeep to base headquarters. He called Enterprises and learned that Bud's condition was improved, and that Mr. Swift had returned that afternoon. He spoke to him about the mystery sub.
 
"This is bad news indeed, son," Mr. Swift said, after hearing how the attacker had defied detection. "You'd better inform Admiral Walter. He had to fly back to Washington."
 
"I'll call him right away," Tom promised.
 
The admiral was equally disturbed when Tom succeeded in reaching him. "We must find that missile as soon as possible—at any cost," he said. "Tom, you Swifts have had considerable experience in undersea dredging. Could you send a team of engineers to assist us in the work?"
 
"Yes, sir," the young inventor replied. "I'll assign men to the job first thing tomorrow."
 
After hanging up, Tom hopped back to the mainland with Chow in a Pigeon Special. This sleek little commercial plane was manufactured by the Swift Construction Company in charge of Ned Newton.
 
Early the next morning Tom and his father drove to Enterprises, and the young inventor plunged into the job of organizing an engineering crew for the missile hunt. Art Wiltessa, a crack underwater specialist as well as engineer, was placed in charge.
 
By noon the group had taken off for the South Atlantic in a Swift cargo jet. A small portable model of Tom's atomic earth blaster was included in their equipment. A jetmarine and a diving seacopter were also dispatched from Fearing to assist in the operations.
 
"It's apt to be a long-drawn-out job—and dangerous," commented Mr. Swift as he lunched with Tom in their office.
 
"Yes. Digging in that silt could be almost as bad as working in quicksand."
 
Mr. Swift's deep-set blue eyes took on a thoughtful gleam. "Speaking of silt, son, I've found the ideal spot for my secret deep-sea farm."
 
"You mean for growing those plants you use in making Tomasite?" Tom asked.
 
The elder scientist nodded. Tomasite, a revolutionary plastic which Mr. Swift had developed, possessed amazing insulating properties against both heat and radiation. One of its secret ingredients came from certain plants found only in Far Eastern waters. Mr. Swift hoped to transplant them locally.
 
"The site is near Fearing Island—about fifty feet in depth," he added.
 
"You may have a tough time finding gardeners, Dad," Tom pointed out. "Men can't work that far down for very long at one time."
 
"It'll be a problem," Mr. Swift conceded. He finished his coffee, then looked up with a twinkle in his eyes. "How about figuring out a solution for me, Tom?"
 
"A new kind of air lung?" Tom was intrigued!