The guide told her they were distributed to certain areas, "You have probably heard that the crocodile is a vanishing species. We are trying to do our part in seeing that American crocodiles do not become extinct."

At this remark Bess heaved a great sigh, "Would America really be badly off if it didn't have any?"

The guide looked at her with contempt in his eyes. "Young lady, if you knew anything about ecology, you would realize how useful they are!"

Bess had no chance to reply because a loud bell rang.

"This is an alarm!" the guide exclaimed. "I must ask you all to get back to the dock as quickly as possible and leave the island!"

"Why?" Nancy asked, disappointed, "Does it mean a crocodile is loose?"

"It could be," the guide replied. "Now please, ladies and gentlemen, return to your boats without delay!"

The tourists ran. Bess was one of the first, and the others, for once, had trouble keeping up with her. Just before they reached the skiff, a guard at the dock asked them if they had registered when they came in.

"No, we forgot," Danny said. "I'll do it now." He hurried into a small office building and signed his name. Then he entered the girls' names as Anne, Elizabeth, and Jackie Boonton.

When he came out again, they had already climbed into the skiff. A man in overalls approached the Pirate Pirate, pulled out a camera, and snapped their pictures, then hurried away and disappeared among the mangrove trees.

Danny jumped into the skiff and pushed off. "Why was that guy taking your pictures?" he asked.

Nancy looked concerned. "I have no idea. He did it so fast we didn't have time to turn our backs or refuse."

"He didn't photograph any of the other visitors," George stated. "He singled us out-for a reason!"

Nancy nodded. "I'm sure the top men here realize who we are. Perhaps they wanted our picture to distribute to the members of the gang who haven't seen us yet!"

George frowned. "This could mean we'll be hara.s.sed by all kinds of people, wherever we go. I'm worried."

"So am I," Bess added. "I think we should return to River Heights and get out of this whole dangerous mess!"

"You don't mean that!" Nancy exclaimed.

"Yes. I do!"

Nancy and George looked at the frightened girl. Finally Nancy said, "If that's what you want to do, Bess, go ahead. As for me, I'm staying right here and seeing this mystery through!"

"So am I," George added.

There was silence for several minutes, then Bess gave in. "You know perfectly well that I wouldn't run out on my friends. But I warn you to be careful. I know I'm not as brave as you arc. I hate to get hurt!"

CHAPTER V.

A Threat.

DANNY and the girls moved away from Crocodile Island. All of them watched anxiously to see if they were being followed.

Other tourists were leaving in their motorboats. Nancy wondered if one of them might contain a spy from the Ecology Company, sent out to pursue the Pirate Pirate.

"I hope not," the girl detective thought.

Nancy observed through binoculars what directions the various craft took. One seemed to stalk the Pirate Pirate, and Nancy had an uneasy feeling about it.

"Do you see anything?" Bess wanted to know.

"There's a fast motorboat called The Whisper The Whisper," Nancy replied. "It seems to be tailing us."

"Is that unusual?" George asked. "After all, other people might be heading for Key Biscayne."

"Of course," Nancy answered. "It's just that most of the boats have scattered. This one stays right in the wake of our boat."

"Oh, oh," Danny murmured. "Maybe they want to find out where we're headed. On the other hand, they wouldn't really have to bother. Anyone can check the boat registry to see who owns the Pirate Pirate and where we live." and where we live."

"That's great," George said. "They already know who we are, and now they can find out where we're staying!"

Bess became alarmed. "Let's head for home!" she begged.

Danny looked at Nancy. "What do you say?"

"I'm not ready to leave Crocodile Island just yet. Let's go around it and see what's on the other side. We might pick up a clue."

Danny nodded. "Sure. We may even find out if a loose crocodile caused the alarm."

"What else would?" George asked.

"You girls. Someone might have recognized not Anne, Elizabeth, and Jackie, but Nancy, Bess, and George, and wanted you off the premises."

Nancy agreed that it was a possibility. "We're getting to be notorious." She chuckled.

Danny followed the sh.o.r.eline of the key. The Whisper The Whisper stayed right behind them, and soon there was no doubt in the girls' minds that the boat was pursuing them. The fast craft finally pulled alongside the stayed right behind them, and soon there was no doubt in the girls' minds that the boat was pursuing them. The fast craft finally pulled alongside the Pirate Pirate.

With some apprehension the young people watched the two men on The Whisper's The Whisper's deck. Both had swarthy complexions and unpleasant faces. One of them shouted, "Get away from this island!" deck. Both had swarthy complexions and unpleasant faces. One of them shouted, "Get away from this island!"

"Why?" Danny asked innocently.

"Because it's private property!"

"The water isn't!" George pointed out "Besides, we're not doing any harm!"

"We don't want you here," the man insisted and shook his fist. "Now get lost!"

"Who are you?" Danny asked. "And why should we listen to you?"

"It's none of your business who we are. And if you don't listen, you'll be in trouble!"

Danny paid no attention to the warning. Instead, he revved up his engine and pulled away from the other craft. Obviously the two men did not know what to do next, so they followed the young people around the island.

Bess had scanned the sh.o.r.eline through binoculars as un.o.btrusively as possible. She focused on a metal tube sticking out of the water. It seemed to give off bursts of light, as if it were studded with prisms and mirrors reflecting the rays of the sun.

"Hey, see that thing over there? I wonder what it is!" she said, excited.

The others looked and George gasped. "It could be the periscope of a submarine!"

"What!" Danny exclaimed. For a moment he forgot to keep his engine racing.

Nancy took the binoculars from Bess and trained them on the strange object. "Is the water deep enough for a submarine to get in?"

"Yes," Danny replied, "Notice that the thing is sticking out of one of the channels where the water is green. That means it's deep enough for a small sub. As a matter of fact, during World War II enemy subs got in here this way. The government had mined all the larger, more important channels to keep them out, but small enemy craft slipped in anyway."

"Danny, can you go into the channel?" Nancy asked. "I'd like to see if that really is a periscope."

"Sure," Danny said and changed course.

But they soon realized that the men on The Whisper The Whisper had no intention of letting them go through with their plan. had no intention of letting them go through with their plan.

"They're coming closer," George said tensely. "Obviously they don't want us to check that thing out there."

"Which proves that they have something to do with it," Bess added.

Again the other boat pulled alongside the Pirate Pirate. "You kids think I'm fooling!" the skipper shouted, "I'm not. If you don't turn around instantly, your boat is gonna get rammed. And it'll cost you a pretty penny to have it repaired!"

"But we're leaving the island," Danny pointed out. "Just as you told us to!"

"You're going in the wrong direction. Turn back!"

Danny hesitated. He realized that this rime their pursuers meant business. Before he had a chance to pull the wheel around, The Whisper The Whisper came so close to their skiff that it sc.r.a.ped the bow. came so close to their skiff that it sc.r.a.ped the bow.

"All right! All right!" Danny cried out, "We're leaving. You don't have to damage us."

The skipper chuckled evilly. "And don't ever come back. You hear!"

Bess had turned white and sat frozen in her chair, her hands clamped tightly around its edge. Nancy and George realized that the situation was critical and did not object to Danny's pulling away in the direction the men had indicated.

The Whisper followed them for a while, then turned off. Obviously the men were satisfied that they had chased the intruders away. followed them for a while, then turned off. Obviously the men were satisfied that they had chased the intruders away.

"Wow!" Bess said finally. "I don't want to see those people ever again!"

Nancy grinned. "I do. They're up to no good, and I'm planning to find out what it is."

As the Pirate Pirate headed toward Key Biscayne, George said, "I wonder who those guys arc. Let's stop at the Coast Guard office and see in whose name headed toward Key Biscayne, George said, "I wonder who those guys arc. Let's stop at the Coast Guard office and see in whose name The Whisper The Whisper is registered." is registered."

"We don't have to do that," Danny said. "My dad has a book containing all the information. Unless it's a brand-new entry, it should be in there."

"I wish we could fee sure that we saw a periscope," Nancy said, still pondering their strange experience.

"How do periscopes work?" Bess asked.

"Oh, I know that because we just had it in school," Danny volunteered. "You see, the periscope is the eye of the underwater craft. A submarine builder by the name of Simon Lake invented the first good periscope, which was way ahead of the technology and science of his time. He bought a lot of lenses and began to experiment."

"Not too complicated!" George said.

"Maybe not, but one day he hit upon a lucky combination. He could look down the street and see people walking and wagons rolling through the harbor. He called it an omniscope. It offered enough magnification and clearness of optics even for night vision, so it was a big success."

"How long ago was that?" George asked.

"Nineteen hundred two," Danny told her. "Before that they just had makeshift equipment."

As soon as they arrived at the Cosgrove house, Danny went to get the boat registry. It was large and heavy. He put it on the dining-room table. The girls peered over his shoulder as he checked "W" for Whisper Whisper.

"Ah. Here it is," he said triumphantly. "It belongs to two men, Matt Carmen and Breck Tobin. They live in Bridgeport, Connecticut."

"Do you know who they are?" Nancy asked.

"No. Never heard of them. I wonder what they're doing down here. They're a long way from home."

"I'm sure they're in league with the men who run the Crocodile Ecology Company," Nancy said.

"Maybe they're supposed to guard the place," George spoke up. "They got rid of us in a hurry!"

"I hope they don't check up on who owns the Pirate Pirate and then come here and bother us!" Bess said, worried. and then come here and bother us!" Bess said, worried.

Danny insisted upon being cheerful about the whole affair. "We may be boxed in, but we're not going to let those guys get the better of us!" he vowed.

Nancy smiled. "That's the spirit! The question is, what are we going to do next?"

When Mr. Cosgrove returned home, the young people told him what had happened and asked his opinion on the case. He thought for a few moments, then said, "Frankly, I'm puzzled. We now have a list of suspicious people, but we still have no idea of what they're up to."

"Or how the periscope fits in," Nancy added.

Mr. Cosgrove smiled. "Are you sure you weren't looking at a marker for a buoy?"

"I don't think so," Nancy replied. "But the only way to find out for sure will be to go back and look again."

"Maybe we shouldn't use the Pirate Pirate," Danny said. "Our enemies are familiar with it. Whenever they see us they'll come after us."

"What do you have in mind?" his father asked.

"Perhaps we could ask our friends the Piarullis if we can use their cabin cruiser." He turned to the girls. "They dock right next to us, and if they're not using the Sampson Sampson, I'm sure they'll let us have it. It's enclosed, too, which would help. Those men couldn't identify us."

"That's a good idea," Mr. Cosgrove agreed. "I'll call them and ask."

He went to the telephone and returned a few minutes later. "Mr. Piarulli said you can have his boat tomorrow. Unfortunately, their son and his wife are taking it up north the following day."