At the strange sound of Tom's voice, Chow jerked around. His eyes bugged out at the look on the young inventor's face. Then he dashed to the public-address outlet on the wall and switched on the mike.

"Help! Help!" Chow yelled. "Tom Jr.'s trapped in his lab!"

The roly-poly chef was quivering in panic. He dashed across the room and paced helplessly about the tank. Within moments, excited men were crowding into the laboratory.

Mr. Swift, among the first to arrive, took in the situation at a glance.

He dashed to the control board and slammed shut the main switch, thus cutting off power to the ion-drive jet.

"Whew! Th-thanks, Dad!" Tom's chest was heaving as he gulped in air to relieve his tortured lungs.

Tom Sr. helped him climb out of the tank.

"B-b-brand my rhubarb rockets," Chow stuttered. "What in tarnation happened?"

"Guess I gunned my new skin-diving jet a bit too hard," Tom said sheepishly. "It was almost a K.O. for me!"

Mr. Swift asked Tom about the invention. After explaining how it worked, Tom added with a grin, "Maybe you'd better hang around, Dad, until I install some sort of density-control gadget for my hydrolung. Then I can go up or down, or stay at any level easily."

Such a device, Tom felt, might prove to be a lifesaver if he should ever become trapped under water--perhaps far from help.

The elder scientist chuckled and threw an arm around Tom's shoulders.

"I'd say you could design something like that with your eyes shut, son!"

Warmed by his father's appreciation, Tom set to work improving his diving apparatus.

An hour later Bud came bursting into the laboratory. "Hey! What's this I hear about your getting hammerlocked by a water jet?" the husky young pilot asked. He had been on a test flight and just returned.

Tom laughed good-naturedly. "Nothing serious. In fact, I felt pretty silly," he told his chum. "I souped up our ion-drive gizmo a bit too much."

Bud picked up the slender metal cylindrical a.s.sembly from the workbench.

"This it?" he asked, his curiosity immediately aroused.

Tom nodded and demonstrated the device in the test tank.

Bud whistled with glee. "Boy! With this rig, we can scoot around like a pair of barracudas!" he exclaimed. "What about that other thing you're working on?" Bud pointed to a small electronic cha.s.sis on the workbench, studded with a tangle of transistors, diodes, and condensers.

"It's a density-control device," Tom explained. "A subst.i.tute for ballast tanks, you might say. It'll enable us to rise or sink to any depth at will, simply by varying our underwater density."

Tom said the device would be carried in a small case, hooked to the diver's belt, with a single tuning-k.n.o.b control. The "throttle" or speed control for the ion drive would be housed in the same unit.

"I can't wait to try out the new diving gear," Bud said excitedly.

By four o'clock Tom had the apparatus perfected, and turned it over to Arv Hanson for fast duplication.

"We'll give it a shakedown tomorrow morning," he told Bud.

The duplicates of the ion drive and density control were ready and waiting when the boys arrived at the plant next day. They immediately flew to Fearing Island and embarked in a motor launch, with Zimby c.o.x again at the helm.

This time they cruised out to deeper water. Tom and Bud donned flippers and belt, and helped each other strap on his ion-drive jet.

"_Down_ we go, into the wilds of sharks!" Bud chortled l.u.s.tily. "Watch your step, Tom."

"Just make sure you come up again in one piece," Zimby said with a grin.

"Also, don't get carried away with that ion squirt gun and take off on a round-the-world underwater cruise."

"Who knows?" Tom joked. Adjusting his face mask, he plunged over the side. Bud followed.

Down they glided into the sea-green wilderness. Leveling off in sight of the ocean floor, they tried their drive jets. The effect was thrilling!

_Zip ... Whoosh!_ They darted to and fro like human torpedoes.

Then Tom twirled the control k.n.o.b of his density unit. Immediately he bobbed upward like a cork. A reverse twirl sent him plummeting toward the bottom again. Bud, watching with wide-eyed excitement, began experimenting on his own.

Soon the boys were engaging in all sorts of underwater acrobatics.

Presently Bud felt a nudge in the back that sent him hurtling a dozen yards through the water.

"Snuck up on me, eh, pal?" he thought with a chuckle. "Okay, Tom old boy, here's where the undersea terror strikes back!"

Swooping around to return the compliment, Bud gulped in surprise.

Instead of his chum, he found himself face to face with a bottle-nosed dolphin!

"Good night!" Bud thought. "A porpoise! So you're the joker who nudged me!"

With a playful toss of its comical-looking snout, the porpoise swam off, as if inviting Bud to join in the fun and games. A whole school of the creatures cavorted into view.

"Okay! If you want to play!" Chuckling, Bud darted in pursuit, whacked the porpoise that had nudged him, and jetted off again. The porpoise gave chase, whistling and grunting audibly.

Tom joined in the fun, and soon a rollicking game of underwater tag was in full swing. The dolphins seemed as playful and mischievous as small children.

Twenty minutes later the boys surfaced and hauled themselves aboard.

Both tore off their masks and flopped into the boat, shaking with laughter, surfacing and diving.

"What was so funny down there?" Zimby asked.

When Tom told him about the dolphins, he too burst into laughter. The porpoises rose into view and convoyed the launch all the way back to the island.

The boys were so jubilant over the performance of the new hydrolung gear that Tom decided to press his search for the Brungarian sea-prowlers immediately. Soon after lunch they took off in the _Sea Hound_ and headed for the South Atlantic. Hank Sterling, Chow Winkler, and two crewmen accompanied the boys.

Dazzling afternoon sunshine sparkled over the sea when they reached the missile search area. Tom immediately contacted Art Wiltessa and the task-force ships. They had no new developments to report.

The young inventor gave orders to submerge. As soon as the seacopter touched bottom, Tom and Bud swam out through the air lock with their hydrolungs.

They probed about for half an hour, ranging farther and farther from the _Sea Hound_. Then Tom felt a touch on his arm. He turned and saw Bud pointing off excitedly to the right.

A strange submarine was moving slowly toward them!