"We'll ask now. How about that octopus cave? Where is it?"

"We'll take you," Rick said. "Let us get tanks from our boat. There should be a pair fully charged by now."

Steve shot a look at the doctor. The officer shrugged, then nodded.

"Okay, if it's a short dive. They've had plenty for today."

"Chief? Where are you?" Kelly called.

A frogman stepped from the rear of the group. "Here, Sir."

"Check their regulators, please. If they're okay, hook up fresh tanks.

If not, loan them complete outfits."

"Yes, Sir."

"Thanks, Chief. Then get set to come with us. Danny, Jonesy, Mike, and d.i.c.k come along, too. Bring still and motion-picture cameras. When we get down, split into two-man teams and search the area. You know what we're looking for. It's just like the one we found off St. Croix."

Rick stared at the frogman officer. Another bra.s.s ball off St. Croix?

But there wasn't time for questions.

"Quick dive, please," Zircon requested. "These boys have had enough."

"They're through as soon as they show us the cave," Steve agreed. "Come on, gang. Let's get to it."

Rick carefully checked his equipment, something that no diver can ever take for granted or leave to someone else, while Scotty did the same.

Then they put the equipment on and adjusted face masks. Their knives, Rick's camera, and Scotty's spear and gun were somewhere near the wreck.

They would have to get them another time.

The group entered the water. Rick looked around and oriented himself by the position of the _Water Witch_, then led the way with Scotty, Steve, and Jimmy swimming along with him while the Navy frogmen stayed closed behind.

It was a thrill for Rick to be swimming with the famous UDT frogmen. He looked to the side and saw that Steve was perfectly at home in the water, and he marveled at the adaptability of the JANIG agent. Steve hadn't been joking when he said he would be an expert by nightfall.

At the reef Rick turned northward and led the way toward the level of the cave. A few moments later he hooted for attention and with pointed finger showed it to Steve and Jimmy. The octopus was still there.

A frogman swam over and picked the little creature up. The octopus spurted away, leaving a blob of ink behind. He came to rest above the cave, poised for further flight.

Rick swam down to the sandy floor of the cave and began to search for the bra.s.s ball. Scotty beckoned, and they swam together toward the spot where Scotty had last seen it. The frogmen swam to the bottom with them, then fanned out, searching.

A few moments later someone hooted, and a tanned, muscled frogman swam over, holding the object triumphantly.

Steve Ames pointed to the surface and Jimmy hooted an order. The group swam leisurely up through the murky layer, oriented themselves by the sleek shape of the destroyer escort hull, and emerged at the landing.

The frogman who had found the ball handed it up to Hobart Zircon.

Steve Ames motioned to one of the frogmen. "Run these fellows over to the beach, please, then wait and bring them back." He turned to the boys. "Put on dry clothes. Then come on back. We need to talk."

An hour later the boys, the scientists, Steve, and Kelly were seated at a table in the destroyer escort's tiny wardroom, noses twitching over the savory steaks that were being served. The boys ate like starved men, talking a steady stream between bites.

Rick sighed and let out his belt. "Well, that's our story. What's yours?"

Steve stirred his coffee thoughtfully. "I can make it short. We don't know the whole story yet, but we will by the time I get back to St.

Thomas. Have you two any idea what these bra.s.s-ball gadgets are?"

"We decided that they were probably sonar equipment of some type,"

Scotty said. "But we couldn't figure out what they were for."

"Easy," Steve said. "Although you couldn't know, of course. They were for spotting submarines."

Rick stared. Submarines?

Steve saw his look of bewilderment. "It happens that our new atomic-powered submarines are conducting manuevers in this area. Does that help?"

It did! Light slowly dawned. "Then these were scanning our subs! But I still don't see why it would be any problem to find them. The subs must have equipment that will tell when sonar beams. .h.i.t them."

"They do. And that's a big part of the story."

Steve sipped his coffee for a moment. "These sonar devices are a new type, and very cleverly designed. They don't send out a continuous beam.

Instead, they operate in bursts, in a random pattern. They might send out a beam twice in a minute, or wait an hour between bursts. The beam is a powerful one. It's effective for an extraordinary distance."

"The wail, of course, was the beam operating," Zircon interjected. "You didn't hear the beam itself, since that's ultrasonic. But you did hear the mechanical vibration of the bra.s.s ball. It had a kind of sub-harmonic effect that was audible."

"That's right," Steve agreed. "Anyway, there were several different stations, in different locations. Some were on islands, some on fishing boats. Since they operated only in short bursts in a random pattern, the subs--and the special teams we sent out--were never able to get a bearing that meant much."

"They must be self-recording," Rick said thoughtfully, "otherwise the enemy couldn't get the information out of them."

"They are. Whatever echo they get makes a tracing inside the box they're mounted on."

Scotty objected, "But what kind of information is it? How can anyone tell anything about the subs from such recordings?"

"By putting all the recordings together and running a rather complex a.n.a.lysis. The a.n.a.lysis will give speed, depth of operation, maneuverability--if the spies are lucky to have beams operating at the right time--and number of torpedoes fired, with the same information on the torps. That's enough information to make it worth an enemy's while."

"I'll say!" Scotty turned to Zircon. "And what were you doing, Professor?"

"I'm afraid I arrived on the scene too late to do much good," Zircon boomed. "However, I believe we can be useful in preventing such occurrences in the future. I have an idea for an improvement in our scanners that will allow a fix to be made on such beams."

There was a pause when dessert arrived. The boys savored excellent apple pie smothered in a generous helping of ship-made ice cream.

"We thought Zircon might help us work out a system of getting fixes on the transmitters," Steve said. "As it happened, we got a lucky break.

The subs happened to have their devices pointed at St. Croix simultaneously when a beam scanned them. They got a fix on it. We flew a team of frogmen down in a Navy amphibian right in the middle of the second storm. They found it, and got the men who were handling it. One of them talked."

Jimmy Kelly picked up the story, "You arrived with Zircon at about the same time the St. Croix team took off from our base, and headed right into the storm. They didn't get back until late last night, and it was nearly dawn before we got the story from the man who talked. Then we loaded on this DE and headed here."

"I'm glad you didn't wait ten minutes longer," Rick said fervently.

"They foxed us. I kept listening for their boat, but they didn't use it."

"They probably decided to swim out and sneak up on you," Zircon stated.

"After capturing Tony last night, their suspicions were probably aroused somewhat. Even if they swallowed his story entirely, it would be only good sense to check up."

Tony looked hurt. "I'm sorry you don't think my story was enough to allay their suspicions entirely. But speaking of listening for their boat, why didn't you hear this ship coming? And why didn't the fancy frogmen?"