"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 assembly 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 chassis on the workbench, studded with a tangle of transistors, diodes, and condensers. "It's a density-control device," Tom explained. "A substitute 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-knob 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 Cox 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 lustily. "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.