For his crew, Bud chose Mel Flagler and another man. Mel was an experienced jetmariner who had gone on the Swift expedition to Aurum City, the underwater ruins of a lost civilization. Here Tom had used his spectromarine selector to restore the ancient buildings. Tom, Hank, and Arv went back to the airfield and soon took off in the diving seacopter. Landing on the water, they submerged and began the undersea detection test. Tom manned the sonarscope personally, eager to conduct as careful a search as possible. "Getting any blips, skipper?" Hank called out from his post at the _Sea Hound_'s controls. "Not a ping, Hank. The system seems to be working out even better than I'd hoped." Tom felt a glow of satisfaction. He explained, however, that the jetmarine's transparent nose pane--which had to be left unprotected for the pilot's visibility--offered one vulnerable spot to sonar detection. "But a little smart maneuvering can cover up that angle," Tom added. "Try the hydrophones, Arv, and see if you can hear 'em." The chief modelmaker slipped on the earphones and listened intently. For another ten or fifteen minutes they probed about with no sound trace of the "invisible" jetmarine. But presently Arv snapped his fingers to catch Tom's attention. "Got her, skipper!" Tom took over the hydrophones. Sure enough, his ears could make out the faint hum of the jetmarine's atomic turbines. Tom directed Hank toward the sound, then ordered him to switch on the _Sea Hound_'s powerful search beam. The light cut a path of radiance through the murky dark-green waters. Dead ahead, the jetmarine could be seen gliding across their field of view. "Your system blinded our sonar okay, skipper," Hank commented, "but this proves she could still be spotted by enemy listening devices." Tom refused to be discouraged. He ordered Hank to return to base and wait for Bud. Meanwhile, the young inventor applied himself to the problem of how to mask the sub's noise. "How about it, pal?" Bud asked, when he reported aboard the seacopter a while later. Tom explained the results of the test and the need for an added safeguard against hydrophone detection. "Think I see a simple way out, though," he added with a pleased chuckle. "Natch! With a brain like yours, it's a cinch," Bud quipped. "Explain, professor." "Well, we can never do away with the noise of a sub's propulsion machinery," Tom began. "That goes without saying. So we'll have to camouflage it--lose it in the underwater jungle noises, so to speak." Bud scratched his head. "How do we do that?" "By amplifying the natural undersea sounds all about it," Tom explained. "Fish and all forms of underwater life make a background noise over the hydrophones, you know." As Bud nodded, Tom went on, "So we simply step up the volume till the sub's own noise gets drowned out or 'wasted' in all the racket."