He brought the Sleuth as near the base of the cliffs as he dared, skilfully avoiding the menacing black rocks that thrust above the water.

Jerry, who had scrambled out on the bow, gestured towards an outcropping of rocks about a hundred yards away.

"Here's our first cave," he announced.

"I remember it," said Frank. "Joe and I went into that one when we were on a car-theft case. It looks like a cave, but is only a few feet deep. No use looking here."

The searchers pa.s.sed several shallow openings, but at last Chet gave a jubilant shout. "Here're the deeper ones!"

They had rounded a little promontory and the boys saw a ragged row of gaping holes in the face of the rock. Most were just a few feet above the waterline.

Chet said, "I know them. Some are small but others are big enough for an elephant to walk through sideways."

Frank brought the Sleuth in still closer to the base of the twohundred-foot-high cliffs.

"Great place to hide someone," Biff commented. "I bet there are hundreds of those caverns."

"We have our work cut out for us," Frank agreed. Some distance on, he spotted the first of the larger holes in the rock. The cave was six feet wide and high above the water. Frank ran the boat in close enough so that by scrambling over its bow one could land on the tumbled heaps of rocks and boulders just beneath the opening.

"Let's take a look," he said eagerly. "Jerry, will you hold the boat here?"

"Sure. Go ahead."

Within a few minutes the others were climbing up the boulders towards the cave mouth. Presently they vanished into the dark interior.

CHAPTER VII.

The Escape JERRY held the nose of the Sleuth insh.o.r.e and manoeuvred so that the propeller remained in deep water He waited impatiently for news of Joe.

It did not take the others long to find that the big cave they had entered was unoccupied. They reappeared a few minutes later.

"Did you find him?" Jerry called.

"No luck," Biff reported.

Chet was discouraged and said so. "We're working on the slimmest of clues," he said. "The fair-haired man and his friends might not have meant the Sh.o.r.e Road caves. Don't forget, there are hundreds of subterranean caverns between Gresham and Bayport."

"But the caves here are the best known," Frank remarked. "Let's look some more. I'll cruise along the sh.o.r.e and pick out the more likely caves to hide a prisoner."

The motorboat edged its way along the face of the cliff: Whenever the boys noticed one of the larger openings that could be reached easily from the sh.o.r.e, Frank ran the boat in among the rocks. Then, while one boy stayed in the Sleuth, the others would scramble up to investigate the cave. The hours dragged by. Finally, they navigated to a place where the cliff sloped and began to give way to sandy hills and wooded inclines.

Biff gave a sigh. "Guess we'll have to give up. There's only one small opening left to investigate."

"But why would kidnappers go up to that cave when there are so many that are easier to reach?" Chet protested. "They'd have to climb fifty feet up to the mouth."

"It isn't as steep as it looks," Frank remarked thoughtfully. "And I can see a winding trail up the slope."

"I'm game," Jerry said.

"Me too," Biff added.

Frank brought the Sleuth in towards the rocks. The boys craned their necks to look up at the tiny opening in the face of the cliff above.

"I guess you're right, Chet," Jerry admitted. "Joe's kidnappers wouldn't climb all the way up there, with so many better caves to pick from."

Chet gave a loud groan. "I've lost about three pounds already, climbing these cliffs."

Despite the worry over Joe, Biff could not refrain from saying, "Then, Chet, you'd better tackle about fifty more caves."

Frank, meanwhile, had seen something that had gone unnoticed by his friends. A piece of newspaper was lodged among the stones under the cave's mouth. The sc.r.a.p of paper might be significant! The fact that it was within a few feet of the cave was suspicious and warranted investigation.

This time Chet volunteered to stand watch and manoeuvred the boat round so that the others could reach the sh.o.r.e from the bow. Frank went first. Biff and Jerry followed.

They climbed the slope, following the trail Frank had spotted. But the incline was so steep and winding that they could make only slow progress in a diagonal direction. The path ended abruptly at a ledge some fifteen feet below the cave. From there they had to climb directly upwards over the rocks.

When Frank reached the piece of newspaper, he picked it up. The sheet was wet and soggy from the fog, but he recognized it as a copy of the Gresham Times, dated the previous day.

His hopes rose with this discovery. Gresham! For the third time since Joe's disappearance the name of that town had come into the mystery! Excited, Frank thrust the paper into his pocket and scrambled up towards the entrance of the cave.

"What did you find?" Jerry demanded, panting.

"Newspaper. It looks like a clue."

Frank reached the cave mouth and stepped inside. The interior was larger than he had thought. Though the entrance was small, the cave widened and seemed to be very deep.

The young detective took a flashlight from his pocket and clicked it. He played the beam on the rugged, rocky walls, the fairly level floor, and finally focused on a wooden box like those used for shipping food.

"Someone's been here!" he shouted eagerly as the others entered the cave. "Look at that box! Fresh bread crusts around it!"

"Don't see anyone now," Jerry observed. "Listen!" The boys heard a peculiar sound, which seemed to come from the back of the cave. The sound was repeated. They listened, staring at one another in surprise.

"Someone's groaning!" Frank exclaimed.

Biff pointed a trembling finger toward a large section of rock about twenty feet away. "From there."

Again they heard groaning.

"Somebody's behind there!" Frank declared.

He ran towards the ma.s.s of rocks and directed the light into the shadows beyond. Frank gasped as its beam fell upon a figure lying bound and gagged on a crude pallet of sacking.

"Joe!" Frank shouted. He sprang forward and removed the gag.

His brother answered feebly, "Frank!"

Biff and Jerry gave a joint yell of delight. They scrambled in behind the wall of rocks and bent over their friend.

Joe looked white and ill. He could scarcely talk to them. His feet were bound together with rope and his hands were tied behind his back.

"To think that we weren't going to search this cave at all!" Biff exclaimed. "And wait until Chet learns we've found you. He's down there guarding the Sleuth."

Frank had already opened his pocketknife and was hacking at the ropes that bound his brother's ankles.

Jerry was working at the other knots.

"I'm hungry," said Joe, when all the ropes had been loosened and he was able to sit up. "I haven't had anything to eat since yesterday noon."

The boys helped him to his feet. "They drugged me," Joe went on shakily, "and I can still feel the effects.

But tell me, how did you find me?"

"Aunt Gertrude gets the credit." Frank quickly told of her encounter with the fair-haired man at Gresham, and his reference to "Hardy" and "caves".

"But Frank put two and two together," Biff spoke up, and mentioned the newspaper clue.

"It was lucky for me you saw the paper," Joe declared. "One of the kidnappers had some food wrapped in a newspaper yesterday. He must have dropped one of the sheets."

"Was the big, fair-haired man really mixed up in it?" Frank asked.

Joe nodded. "He was in it, all right. But there were others. They were after that secret in our car. It's a long story. Let me tell you about it later."

The boys refrained from asking more questions, "Do you feel strong enough to come with us now?" Frank asked.

Joe, with a flash of spirit, started to walk. He wavered for a moment and would have fallen if Frank had not caught him.

"If you can't make it, we'll carry you," Jerry offered.

Joe shook his head and sat down weakly. "My legs are so numb from being tied up, I don't seem to have any strength in them. I'd better wait a few minutes."

At that moment they heard a loud noise. It was a clattering, rolling sound, as if a rock had been dislodged and gone tumbling down the steep incline.

"What was that?" Biff whispered.

Joe got to his feet. "My captors are coming back! Quick! We'll have to clear out!"

"Can they get in here through the rear of the cave?" Frank wanted to know.

"Yes, a pa.s.sage leads down from the top of the cliff."

Frank and Jerry each slipped an arm round Joe's shoulders and helped him towards the mouth of the cave. Biff ran on ahead.

When Chet saw Joe, he gave a war whoop of joy. The others motioned frantically for silence, but their jubilant chum did not understand their urgent signals. He proceeded to put on a noisy celebration. He yelled, waved his arms, and then, to their horror, began whistling shrilly.

The men coming down the pa.s.sage into the cave would certainly hear the commotion and hurry to investigate. The boys must flee quickly!

Frank and Jerry scrambled down the slope with Joe. They reached the first ledge in safety, with Biff slipping and sliding along the path ahead of them. As they commenced the second half of the descent the boys heard a yell behind them.

Frank looked back. A man was standing at the mouth of the cave. He glared at the boys a moment, then turned and shouted to someone behind him. Two other men quickly joined him.

"Go on!" Joe cried. "I'm holding you up! If they catch us, we'll all be in trouble."

"Leave you, my eye!" Jerry growled.

By this time Biff had nearly reached the boat. He called out to Chet, who apparently had not seen the men in the mouth of the cave. At Biff's warning, Chet stopped his noise. Frank, and Jerry clung to Joe on the narrow path, with loose rocks sliding treacherously beneath their feet.

Frank glanced back again. One of the men had drawn a revolver from his pocket and was pointing it at them. Another had stooped and was s.n.a.t.c.hing up stones.

The revolver barked. A bullet whistled overhead. Frank and Jerry ducked and almost lost hold of Joe. A heavy stone hurtled past them and splashed into the water beside the boat.

A hail of stones followed. The man with the revolver fired again and again and several bullets came dangerously close to their mark.

Chet had revved up the engine, ready to take off as soon as his pa.s.sengers climbed aboard.

"Hurry!" Biff yelled. "Only a few yards more!"

Frank and Jerry scrambled to the bottom of the incline with Joe. One of the three men was stumbling down the path in pursuit.

Jerry leaped on to the bow. With Frank on the sh.o.r.e and Jerry helping from the boat, Joe was hauled aboard. Frank was about to jump on to the bow when he felt a heavy, sharp blow on his left leg. He lost his balance and fell partly into the water. When he tried to rise, his leg doubled beneath him. One of the rocks hurled by the men had found its mark!

Shots sounded again. A splinter flew from the bow of the boat.

"Hurry, Frank!" Chet urged.

"Give me a hand," Frank said grimly.

Biff scrambled over the side, seized Frank, and laid him on deck. Frank's leg throbbed and he could scarcely keep from crying out.

The man on the path was only a few yards away now! He showered the air with rocks!

CHAPTER VIII.

An Astounding Report SMACK! A large rock hit the water with a resounding crash only inches from the Sleuth. A deluge of spray drenched the boys.

Chet, at the helm, could hardly see. Wiping the water from his eyes, he revved the motor and took off.

The Sleuth made sternway from sh.o.r.e.