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 pa.s.s 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 a.s.sist us in the work?"

"Yes, sir," the young inventor replied. "I'll a.s.sign 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 a.s.sist 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!

CHAPTER IV

AERIAL ATTACK

"Yes, son," Mr. Swift went on. "What's needed is a new type of breathing device--one that will eliminate bulky air tanks and permit a skin diver to stay down for long periods."

"Quite an order, Dad."

Grabbing a pencil, the young inventor began sketching. In both his Fat Man suits and his osmotic air conditioner, Tom had already perfected ways of drawing oxygen from sea water.

"But a small gadget for skin divers," he said, "will take a fantastic job of electronic miniaturization." After a pause he added, "It could really speed up recovery of the Jupiter prober, though."

Lunch over, Tom hopped a jet scooter and sped off to his private laboratory. The modernistic gla.s.s-walled structure--designed by Tom himself--had every tool of modern scientific research, from electronic microscope to helium cryostat.

As always, whenever he was absorbed in a new idea, Tom was eager to get to work. "Let's see what I'm shooting for. A small container, slung around the diver's neck?... No, too dangerous. Better hook it to his weight belt, with a tube to his face mask."

Using a plastic foam "breadboard," Tom began experimenting with various circuit designs. He worked through the afternoon and returned to the problem early the next morning.

He was interrupted by a message from Art Wiltessa, reporting no luck so far in finding the missile. Later, shortly before lunch, Tom received another call, this time from Admiral Walter. "Just wanted to keep you posted, Tom. Our task force reports no success on their part in finding the buried missile. No sign of the enemy, either."

"They'd probably hesitate to attack any official U.S. Navy units," Tom said. "Or it might mean they've already found the missile themselves."

"That's what I fear," Admiral Walter confessed gloomily. "However, we'll continue searching."

Tom promised to fly down to the site at the first opportunity, saying he was developing a new device that might a.s.sist in the search. After s.n.a.t.c.hing a hasty lunch, Tom returned to work.

Arv Hanson machined several parts and molded the plastic face mask to Tom's specifications. By evening the new device was completed.