as the sea hound returned to fearing island from its search for bud's jetmarine, tom was beside himself with worry. had his experiment cost the lives of his best friend and the other crewmen aboard?

"i'll never forgive myself if anything's happened to them!" tom muttered bleakly.

hank sterling squeezed the young inventor's arm. "you know bud's high spirits, skipper," he said. "he may have taken off on some crazy lark."

"sure! a whale hunt, maybe!" arv hanson wisecracked, trying to lighten the gloom.

tom forced a grin, but he remained heavy-hearted as they neared the base. his only hope now was that a radio message from the jetmarine might have been picked up while they were gone.

as soon as the seacopter was moored, tom leaped ashore. the crewmen on the docks had no news to report, so tom piled into a jeep with arv and sped off to the fearing communications center. hank remained aboard the sea hound to secure all gear.

churning along the graveled road, tom and arv passed the launching area. huge, needle-nosed cargo rockets and the mighty spaceship titan loomed against the sky. tom's moon-voyaging challenger and his more recent space craft the cosmic sailer were also based there.

"going to alert the navy for a search?" arv inquired as they reached the communications building.

tom nodded and braked the jeep to a screeching halt. "coast guard too. they can pass the word to commercial shipping to be on the lookout."

a telephone rang as he hurried into the office.

"for you," the clerk said, looking up at tom. "nice timing!"

tom grabbed the phone. his face widened into a grin. "bud! you seagoing jet stream! what happened?"

arv grinned, too, in relief.

"your antidetection gear worked so well we vanished right out of the ocean!" bud replied with a chuckle. turning serious, he reported how his jetmarine had trailed the mysterious intruder and how he and mel had captured the two brungarian frogmen and their shore contact.

"nice going, pal!" tom exclaimed.

"but here's the catch," bud went on. "when we took off again in our hydrolungs to go back aboard ship, the jetmarine was gone!"

"maybe she's trailing the enemy sub," tom conjectured.

"that's what i'm hoping," bud said uneasily. "trouble is, our subs aren't armed, and who knows about that brungarian job? the way they sling missiles around, anything could happen if she spots the jetmarine."

tom frowned. "i'll organize a search right away. where are you calling from?"

"police headquarters at sandbank."

"okay. take it easy, and i'll send a whirlybird to pick you up," tom promised.

"and don't forget some clothes," bud added with a chuckle. "mel and i are getting chilly."

"right!" tom hung up and gave arv hanson a quick briefing.

then he phoned the base airfield to dispatch a helicopter. he also contacted the nearest coast guard station and put through a long-distance call to navy headquarters in washington to request help in searching for the jetmarine. finally he and arv headed back to the submarine docks in the jeep.

a flurry of activity followed as tom detailed ships for the search and rounded up crews. he was interrupted by a phone call in the loading shed. it was the control-tower operator.

"one of our drone planes has spotted a sub approaching, skipper," the operator reported.

"what bearing?" tom demanded excitedly.

"one-seven-six." tom was about to hang up and grab a pair of binoculars when the operator added hastily, "wait! it's responding to our radio challenge!... that's ours, all right!"

tom dashed out of the shed and scanned the sea to the southward. sure enough, a jetmarine had surfaced and was speeding toward the sub docks. minutes later, tom was shaking hands warmly with zimby cox and mack avery.

"is bud okay?" was zimby's first question.

"right! i just heard from him," tom replied. "he and mel captured those enemy frogmen and a copter's on the way to pick them up. what happened to you fellows?"

zimby confirmed bud's guess that they had taken off in pursuit of the enemy craft.

"we figured bud and mel could make out on their own," zimby explained. "and we thought the sub's course or actions might tip us off to its nationality. also, if it tried any sabotage or mine-planting, we could radio the navy."

instead, cox went on, the mysterious craft had proceeded to a point about ten miles offshore where it rendezvoused with another submarine.

"and get this, skipper!" mack avery put in. "the other sub was undetectable! we were close enough to get a peek at it, but we couldn't ping it on the sonarscope."

"that figures," tom said grimly. "those frogmen were apparently brungarians."

zimby cox related that a man had transferred from the undetectable submarine to the one they had been following. the first sub had then headed out to sea, as if to cross the ocean back to its home base. the other had departed on a course toward the south atlantic.

"probably back to the lost missile area. at least that's the way we figured it," zimby added.

"and neither sub spotted you?" tom questioned.

zimby grinned wryly. "we might not be here if they had detected us. but i'm pretty sure they didn't. anyhow, they gave no sign."

tom was doubly elated at the news. his antidetection gear had evidently worked perfectly in a showdown test with the enemy, even at close range. moreover, if the second sub was returning to the south atlantic, it seemed likely that the enemy, too, had not yet located the precious missile with its data from jupiter.

"you guys rate navy medals," tom told zimby and mack jubilantly. "come on back to shopton with me and i'll buy you the juiciest steaks in town!"

before leaving the base, tom called the coast guard and the navy to cancel his search request. he also telephoned a full report on the enemy submarines to admiral walter.

after hanging up, tom decided on another move. "our antidetection gear seems to have panned out pretty well," he told hank. "i think we should make use of it right away. by sending that jetmarine to the south atlantic, we might get a line on enemy activities down there."

hank was in favor of the idea. he volunteered to prepare the jetmarine for a cruise and take off from fearing that very night.

"thanks," tom said with a parting handshake. "keep us posted if you learn anything."

meanwhile, bud and mel flagler had arrived at the base by helicopter. they and their two shipmates flew back to the mainland with tom and arv for a celebration dinner in town.

the next morning found the young inventor hard at work in his private laboratory. he was tapping his head with his slide rule and frowning at a blackboard scrawled with equations when bud dropped in for a visit.

"what now, inventor boy?" his copilot asked. "don't you ever give that brain of yours a rest?"

"oh, hi, bud!" tom looked around absent-mindedly. "i'm just trying to figure out a way to crack the brungarians' antisonar system."

"good night!" bud sank down on a lab stool. "you've come up with a way to make our own subs undetectable. isn't that enough?"

tom shook his head. "not if we want to keep track of those sneaks. and i think i see a way to do it."

"how?"

"so far, i have been thinking about refining our own search sonar." tom explained that the new system he had in mind would send out a complex pulse—that is, an underwater sound wave with many harmonics instead of a single tone, sharp-peaked sound impulse.

"this will make it less likely that their antidetection gear will absorb all of it," tom went on. "what's not absorbed will return as an echo. i'm also going to modify our receivers. but i've still not worked that out."

bud nodded, his forehead puckered in a look of concentration. "so—?"

"so our sonar picks up all that hash, and by means of a computer setup filters out the sub's real echo from the shadow reflections."

"hey! sounds pretty cute," bud said.

tom broke into a dry chuckle. "right—if i can do it." after that job, tom added, he hoped to adapt his own antidetection methods to make hydrolung wearers safe from underwater detection. "and if the jupiter prober hasn't been found by that time, bud, i'm going to request the navy to let us take over the search alone."

bud gave a whistle of excitement at the possibility of new undersea adventures ahead. "count me in, pal!"

the two boys broke off their conversation a short time later and went back to the administration building for lunch with tom's father.

mr. swift greeted them with a smile as they entered the big double office. "glad you could join me, boys! chow's laid out quite a feast for us today."

three places had been set at the conference table, and an appetizing repast of sizzling ham and sweet potatoes waited in covered dishes on a lunch cart nearby.

"mmm!" bud inhaled the aroma. "good chow from good old chow!"

tom switched on the videophone screen to a private channel to catch the noon news while they ate. the newscaster wore a look of excitement as he spoke without pausing for the usual commercial.

"the brungarian government has just scored a propaganda bombshell!" he reported. "in a news announcement released less than half an hour ago, they stated that their navy has perfected an undetectable submarine!"

the swifts and bud froze, openmouthed, at the newscaster's words.

"no need to tell you what this could mean to american security," he went on. "if enemy subs slipped through our continental defenses, their missiles could devastate the united states with scarcely an instant's warning! the whole country's been rocked by the announcement. an official comment by our defense department is expected at any moment."

"sufferin' satellites!" bud gulped.

mr. swift nodded. "it's a great propaganda stroke. but i wonder why they've chosen to reveal their secret at this time."

tom said thoughtfully, "dad, do you suppose they've realized the fact that we know about their antisonar gear?"

"could be, son. they may figure that since the secret is out already, they may as well play it up for all it's worth." the elder scientist paused and frowned. "or it might be intended to force our hand."

"you mean in hopes of getting us to reveal whether or not we have an antidetection system ourselves?" as his father nodded, tom scowled. "if so, that sub yesterday may have been observing our tests."

the telephone rang and tom leaped to answer it. the caller was dan perkins of the shopton evening bulletin.

"you can guess why i'm calling, tom," the editor said. "how about a statement from you swifts on this brungarian sub story?"

"we found it very interesting," tom said politely but noncommittally. parrying further questions, he hung up as soon as possible.

mr. swift approved tom's policy of silence. almost immediately the phone began ringing again with a succession of calls from other newspapers and wire services. tom dashed off a brief, general statement and instructed miss trent to give it to all further callers.

"maybe this is a good time to make a private announcement to you fellows," mr. swift said to the two boys, his eyes twinkling. "do you recall my telling you that doc simpson had isolated an unknown vitamin from the space plants? well, we've now discovered that this vitamin can condition the human body to stay under water indefinitely. doc is putting some up in capsule form."

both tom and bud gave whoops of glee at this news.

"dad, you've helped overcome one of the big problems in our search for the lost missile!" tom exclaimed.