the boys exchanged looks of fear through their face masks as the knifelike hull and conning tower of the submarine loomed gray and ghostly.

was the sub brungarian? and what was it up to? were the two young skin divers about to be run down or kidnaped?

or was its crew friendly?

"better not chance it," tom decided fast. he caught bud's eye again and motioned upward with a jerk of his thumb. "topside, pal!"

"roger!" bud's lips shaped the word silently behind his face mask.

in a twinkling both boys flicked their density controls and zoomed upward. the sub at once seemed to betray a hostile intent. it blew its tanks and planed upward in pursuit. but tom and bud easily pulled away. their density units worked like magic, shooting them straight toward the surface.

"wow!" bud shoved back his face mask as they broke water. "that baby was after us and no mistake!"

tom nodded, treading water. "let's not stick around here, either! we'll soon have company again if we do!"

bud did not argue. "where to, skipper?"

in the fresh salt air, with the sunshine sparkling on the waves, it was hard to believe that an enemy submarine was hot on their trail. but both youths realized their peril was growing by the moment.

"back toward the sea hound," tom said, pointing north-northwest. "submerge as we go!"

bud circled his thumb and forefinger, then adjusted his mask, and the two boys plunged back in. on a sloping downward course, they sped along like undersea rockets, their ion jets functioning perfectly. minutes later, they sighted the seacopter.

hank waved to them through the cabin window as they glided past. the air lock opened speedily and the two boys entered. both heaved sighs of relief when they were safely inside.

"somethin' wrong?" chow asked, sniffing trouble.

"a strange submarine," tom reported. "brungarian more than likely. it may be heading this way if they've tracked us."

"a sub?" hank was startled. "we've picked up nothing on sonar!"

"check again," tom ordered.

the sonarman bent to his scope and hank listened intently over the hydrophones. neither could detect any sign of another craft.

"probably the same one that fired on us the last time," tom said grimly. "we'd better clear out before they take another pot shot at us."

hank sent the sea hound zooming toward the surface while the boys changed quickly into slacks and t shirts. then tom took over the controls for the flight home.

"brand my vitamin vittles! are we just goin' to turn tail an' run every time them varmints come skulkin' around?" chow fumed as the seacopter arrowed northward.

"not if i can help it," tom vowed. "but first i must figure out a way to make our own craft invisible, so to speak. it's the only way to protect our american crews, chow, if we hope to do any secret digging for that lost missile."

"want another suggestion, skipper?" bud put in. "this one is about the hydrolung."

"sure. speak up."

"how about putting some sort of communications system into our hydrolung gear? if i hadn't been close enough to grab you when i spotted that sub, it might have been curtains, pal!"

"you're right," tom agreed. "i'll get to work on it."

it was sunset when fearing island came into sight. the boys flew a pigeon special back to enterprises, where tom phoned a full report on the mystery sub to the navy department. then the two chums drove to the swift home for a late supper.

phyl newton was visiting sandy that evening, but the girls displayed a marked coolness toward tom and bud. instead of engaging in conversation, they retired to sandy's room upstairs to play records, while mrs. swift served the boys a warmed-up but tasty meal of roast beef and mince pie.

"what's wrong? are we repulsive or something?" bud asked as they ate.

tom shrugged, concentrating on a mouthful of roast beef. "search me. we sure don't seem very popular with the girls tonight."

mrs. swift, overhearing their remarks in the kitchen, smiled but maintained a diplomatic silence.

suddenly bud slapped his forehead. "good night! no wonder!"

tom looked up with a grin of interest. "well, what have we done?"

"it's what we haven't done, pal!" bud retorted. "we had a date this afternoon, remember? that beach party and dance put on by sandy and phyl's school sorority!"

tom gulped. "oops! boy, we really did pull a boner this time! i completely forgot!"

as they finished supper, the boys discussed various ways to make amends. boxes of chocolates? flowers? none of their ideas seemed to have the proper spark.

"we'll have to come up with something super," bud said.

"right!" tom agreed. "let's sleep on it and see if we can't dream up something by tomorrow morning that'll really wow them."

the next morning tom had a flash of inspiration as he drove to the plant in his sports car. he hailed bud at the first opportunity.

"i have it, pal! what say we stage an old-fashioned square dance tuesday night at the yacht club on lake carlopa?"

bud's eyes lighted up. "hey, that's a great idea! we'll invite a whole gang, get chow to handle the refreshments, and make it a real shindig!"

the boys shook hands enthusiastically. eager to patch matters up as soon as possible, they invited sandy and phyl out to lunch that day. over dessert, the boys announced their plans for a square dance.

"we—uh—realize we goofed yesterday on that beach party," tom said sheepishly. "but we're hoping you'll give us another chance."

the girls looked at each other, their eyes twinkling, then burst into giggles.

"you're forgiven completely!" phyl declared.

"then it's a date?" bud put in.

"you bet it's a date, and don't you forget it!" sandy warned. "phyl and i are going right over to dorman's department store and pick out some cute outfits for the dance!"

tom and bud chuckled over the success of their scheme as they drove back to enterprises. later that afternoon a telephone call interrupted tom as he worked in his lab on a sonic-communications system for the hydrolung apparatus.

"this is lester morris," said the voice at the other end of the line. the name did not register with tom at first until his caller added, "i hear you're planning a square dance tuesday night at the yacht club."

suddenly tom remembered. lester morris was a popular dance orchestra leader in and around shopton. he was also much in demand as a square-dance caller and fiddler.

"that's right," tom said with a chuckle. "news must travel fast. we just phoned invitations to our friends."

morris asked if musicians had been hired for the evening. when tom said no, his caller volunteered for the job, offering to provide a small combo of country-style players. his asking price sounded like a bargain rate, and tom, knowing morris's reputation, was only too glad to engage him.

"lucky break, his calling," the young inventor thought as he hung up.

bud was delighted to hear of the arrangement when he came into the laboratory a while later. the boys talked over their dance plans for a few moments, then bud asked:

"how's our underwater talkie system coming?"

tom scratched his jaw thoughtfully. "a bit tricky but not too difficult," he replied. "it's mostly a job of adapting the sonarphone arrangement from our fat man suits—in miniature."

a tiny mike, tom explained, would be installed on the inside of each face mask, with its output feeding to a sonar transducer on the exterior. the receiving transducers would feed from amplifiers to earphones. the hookup would be powered by the solar battery in the hydrolung power unit, by connecting wires through the breathing tube.

"that's neat, tom," bud said. "need any help?"

"you can mold us a pair of new face masks—big enough to cover the earphones," tom suggested. he handed bud a penciled sketch from the workbench, adding, "then drill the holes for the mikes and earphones—the dimensions are there on the drawing. but watch it so you don't crack the plastic."

while bud complied, tom began assembling the tiny electronic parts. in two hours the gear was ready for testing.

tom wiped his perspiring forehead and gave bud a grin of satisfaction. "go get your swim trunks, fly boy. let's give it a tryout in the tank."

"swell idea! be back in a jiff!"

after a quick change, the boys strapped on the new hydrolung equipment. before adjusting his face mask, tom mentioned that he had inserted scrambling circuits into the communicators to foil any enemy eavesdroppers.

"if they do pick up anything, it'll sound like chop suey," tom ended with a chuckle.

the boys submerged in the test tank and proceeded to give the new underwater communication system a thorough check-out. it worked perfectly. ten minutes later tom and bud clambered out again, dripping wet but well satisfied.

they had just peeled off their masks when chow came charging into the lab, with a crowd of workmen and technicians at his heels. the cook was wild-eyed with panic.

"what's wrong, chow?" tom asked in alarm.