"don't worry, phyl. it may be only a blown fuse," tom tried to assure the fearful girl.

but tom was worried himself. not only might he be in danger, but it could involve his friends!

nevertheless, he raised his voice above the excited babble. "please be calm, everyone! we'll have the lights on again in a jiffy!"

taking phyl by the hand, tom groped his way toward the main door.

"let's check the switch," he murmured, and ran his hand over the wall near the door. he located the metal plate and flipped the switch.

the lights went on! good-natured cheers arose. bud, grinning but puzzled, left sandy's side long enough to come over and speak to tom.

"what happened?"

"i guess some practical joker clicked off the switch."

bud suddenly caught sight of a stout youth in a plaid shirt and blue jeans, who was standing in a nearby corner. he was shaking all over with half-stifled merriment.

"there's the wise guy! rock harriman!"

rock, an all-star tackle on the shopton high football team, was well known for his pranks and practical jokes. bud rushed over.

"okay! confess!" the husky young flier roared in a jokingly ferocious tone.

"don't get sore!" rock gasped between chuckles. "i couldn't resist. boy, did you hear everyone squeal when the lights went out?"

tom grinned in relief. "how about another dance, phyl?"

as the music struck up again, he squeezed phyl's hand. "i sure appreciate your concern, even if i didn't rate it."

phyl blushed as she returned the squeeze. "you rate with me," she confided shyly.

the festivities finally ended after a thoroughly enjoyable evening. both sandy and phyl declared to their dates that it more than made up for the forgotten beach party.

"but let's not wait too long for the next date," sandy warned playfully.

"okay, that's a deal," bud promised.

the next morning at the plant tom called on harlan ames. he told of the sinister hoax by the caller who had passed himself off as lester morris. the security chief promised to investigate.

"i'll tip off the police about len unger," ames added. "if they can find him, we may be able to crack this case wide open."

tom telephoned bud, hank sterling, and arv hanson to meet him at the helijet hangar. the four took off in one of the swifts' whirling ducks, which was standing by loaded and ready. soon they landed on fearing island, where tom would try out his antidetection invention.

"what'll we use for a test sub, skipper?" hank asked as they drove toward the docks.

"a jetmarine," tom replied.

a truck with engineers and technicians was following the jeep. it carried the equipment which tom and bud had assembled the previous day.

when they arrived at the docks, tom gathered the men in a loading shed. he showed them his drawings and explained how his "sonar-blinding" setup would operate.

"don't let the diagrams fool you. the basic idea is very simple. we absorb all sonar impulses that hit the ship and transmit them out the opposite side of the hull, instead of letting a ping bounce back and show up on the sonarscope of any hostile sub on the lookout for us."

most of the job, he went on, would be tedious detail work. it would consist of attaching hundreds of mikes and speakers all over the hull to pick up and transmit the sonar pulses. the mikes would be receiving transducers and the speakers would be transmitting transducers.

"the leads from them," tom ended, "will be centralized in a single electronic control unit inside the ship. i'll handle that part of it."

"great idea, tom!" arv hanson said admiringly.

"but what a job it'll be rigging those transducers," put in one of the technicians.

tom nodded wryly. "you're right, danny. if this experiment works out, though, i think i can lick that problem on future installations."

the young inventor explained that he hoped to find a way to mold the transducers into a continuous plastic sheet. this could be applied to the hull of a submarine in a single operation.

"but this time we'll have to do it the hard way," tom added with an apologetic grin.

a jetmarine was hoisted into drydock and the work crew swarmed over it, rigging the transducers. would his experiment succeed? tom wondered. hopefully, he set to work assembling the electronic control unit.

bud helped the men on the hull for a while, then descended through the hatch to see how tom was progressing.

"i'd go gaga trying to keep track of those circuits," bud said, as he watched tom installing the delicate transistors and other components with an electric soldering gun.

the young inventor grinned. "it'll be simple enough when the control unit's all put together," he replied. "just a single on-off switch and one test circuit."

by noon, after working at a frenzied pace, the job was done. tom thanked each one of the men personally. then everyone went to eat lunch.

after the meal, hank sterling asked, "how about a detection test to see how she works?"

"coming right up," tom said. "want to skipper the jetmarine, bud?"

"sure do!"

"okay. pick out a couple of men for a crew and take her down." tom produced a hydrographic chart of the waters around fearing and marked out a test area. "cruise around there for an hour and we'll try to spot you in the sea hound."

"hide and seek, eh?" bud grinned and snapped a salute, then left to supervise the relaunching of the jetmarine.

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."

this could be done, he concluded, with fairly simple amplifying equipment. bud, hank, and arv were jubilant at the idea.

"nice going," bud said. "how soon can we give it a try?"

"soon as i can rig up the amplifier," tom promised.

in less than two hours they were ready to submerge again. zimby cox joined the crew. bud suggested taking along hydrolungs in case of any need for tinkering with the transducers or amplifying equipment.

this time, the jetmarine scored perfectly on the test, successfully eluding all the sea hound's efforts to detect it. tom returned happily to base, feeling that the antidetection problem was now solved. the jetmarine, however, failed to appear.

"that's funny. the test was over at four-fifteen," tom murmured.

"maybe bud surfaced out at sea somewhere," arv hanson suggested.

repeated radio calls brought no response. tom, now seriously worried, took the seacopter down again for another search, hoping that bud would have switched off the antidetection gear by this time. but neither sonarscope nor listening devices revealed the slightest clue.

tom, hank, and arv exchanged fearful glances. had the jetmarine foundered on the ocean bottom—perhaps fouled somehow by tom's new invention? or had bud and his crew fallen victim to the enemy?