The boys quickly roped themselves together at their waists, and Pete led the way down the narrow path. Below, the sea surged in and out among giant black rocks made silvery by the moonlight. At low points on the path, spray flew up to drench the boys. Three times they had to turn and face the cliff to inch across narrow ledges.

At last the path led sharply downwards and they found themselves on a small white-sand beach. The beach was deserted now, but there was considerable evidence that people had been swimming there — empty beer cans, soft-drink bottles, and the remains of picnic lunches.

“We’ll look all along the cliff for some form of opening,” Jupiter decided.

The cliff face was heavily overgrown with scrubby bushes and stunted trees, and hidden in many places by large boulders. With their lights the boys searched behind the bushes and boulders. But they found no entrance to the cave.

“I think we’re looking in the wrong place, Jupe,” Pete said.

“Where else is there to look?” Bob asked.

“Well,” Pete explained, “no one has told us about another entrance. If there is one, I’ll bet it’s hard to get to.”

“You mean it’s not here on the beach?” Bob asked. “But it has to be near here because that path is the only way down.”

“I think you’re right,” Jupiter said. “Bob, you come with me. We’ll search on the right side. Pete, you go left.”

The rocks bordering the beach were slippery with seaweed and mussels so Jupiter and Bob had to pick their way cautiously. Jupiter kept his flashlight aimed at the cliff face in order to look for an opening.

Finally, they reached a point where they could go no farther unless they plunged into the water. Discouraged, they were turning back when they heard Pete shout.

“I found it!”

Bob and Jupiter scrambled over the wet rocks and ran pell-mell down the beach. At the far end, Pete was standing on a big flat rock. Between two giant boulders, they saw an opening in the cliff face. It was a small opening, and only a foot above the sea.

“I can hear the moaning again,” Pete said. “Listen.”

There was no mistaking the sound.

“Aaaaaahhhhhh — oooooooooo — ooooo — ooo!”

It floated out of the opening, very faint as if from deep inside the cavern.

Pete shone his flashlight into the entrance. It was black, wet and very narrow. The tunnel seemed to bore directly into the cliff face as far as they could see.

“It’s awful narrow and dark, Jupe,” Pete said uneasily.

“Maybe it doesn’t go anywhere,” Bob speculated.

“No,” insisted Jupiter. “It must go into the cave or we wouldn’t hear the moaning.”

“It sure looks small,” Pete said doubtfully.

Jupiter squatted and peered into the opening. “I think we can enter safely if we’re careful. Bob, you’re the smallest so we’ll tie the rope to you and you’ll go in first.”

“Me? All alone? I thought we were all going to go in together.”

“That would be the wrong way to do it, Bob,” Jupiter explained. “When entering an unknown passage, the correct way is to send one person in, with a rope on him, while the other two remain outside ready to pull him out instantly if he encounters any danger.”

“Sure,” Pete added. “In those prison camp films, when the soldiers dug tunnels to escape, they always tied a rope to the man who was inside the tunnel. If he pulled once on the rope they hauled him out.”

“Exactly,” Jupiter said with a trace of annoyance. The First Investigator did not like to have anyone suggest that his ideas were not original. He turned to Bob. “Remember, pull hard on the rope if you get into any trouble. We’ll pull you out.”

Somewhat unconvinced but game, Bob tied the rope firmly around his waist. Cautiously he crawled into the narrow tunnel.

Inside it was dark and cold. The ceiling was much too low for him to stand up, and the walls were wet and slimy with green sea moss. He had to inch along on his hands and knees. As he crawled slowly forward, beaming his flashlight ahead, crabs scuttled away, their claws scraping on the damp rock.

After Bob had gone about thirty feet, the ceiling suddenly inclined sharply upward. He stood up. His flashlight beam showed that the tunnel still led straight ahead, but it had become wide and dry, and angled upward.

“Jupe! Pete! It’s okay,” he called back into the tunnel.

Soon both Pete and Jupiter were standing beside him.

“It’s all dry here,” Pete observed.

“This part must be above the high-tide mark,” Jupiter said. “I’ll start marking our trail, and you and Bob listen for the moaning so we can go in the right direction.”

They moved ahead, Jupiter stopping every ten feet or so to leave question marks in white chalk. After some forty feet they emerged into still another of the vast caverns that seemed to honeycomb the interior of Devil Mountain. Once more many dark passages led out of the large room.

The boys looked at each other in dismay.

“Well, here we go again,” Pete said.

“This mountain is nothing but tunnels,” Bob sounded discouraged. “How will we ever trace the sound?”

But Jupiter was not really looking at the new cavern or the many tunnel exits. He was listening.

“Have either of you heard the moaning since we came in?” Jupe asked.

Bob and Pete looked at each other.

“Gosh, no,” Bob said.

“Not since we were outside!” Pete agreed.

“I didn’t hear it all the time I was crawling,” Bob added.

Jupiter nodded thoughtfully. “As soon as we enter, the moaning stops. A most suspicious circumstance. Not once, which could be an accident, but twice now.”

Pete was puzzled. “You think maybe we do something when we come in? I mean, maybe we change something without knowing we change it?”

“That’s one possibility,” Jupiter acknowledged.

“Another explanation might be that someone sees us,” said Bob. “But how could anyone have seen us on that beach in the dark?”

Jupiter shook his head. “I admit I’m at a loss to explain it myself. Perhaps it is just — ”

They all heard the sound at the same moment. A faint and distant jingling of bells, and the clip-clop, clip-clop of a horse’s hoofs.

“A horse!” Bob exclaimed.

Jupiter turned his head and listened intently. The sound seemed to come from the wall of the cavern.

“It’s… inside the mountain!” Jupiter said.

Bob protested. “It can’t be, Jupe. It has to be from another part of the cave.”

Jupiter shook his head. “If my sense of direction is correct, the other part of the cave is to our left,” he said. “We are facing the side of the mountain — and no tunnels go in that direction!”

“Maybe we better get out,” Pete suggested.

“I think,” Jupiter said hurriedly, “Pete is right. Let’s go!”

The boys pushed pell-mell against each other in their scramble for the narrow exit. Pete reached the small tunnel first and began to crawl through. Jupiter and Bob followed right behind.

They tumbled out into water up to their knees, and floundered across the flat rocks till they sprawled at last on the white sand of the beach. They lay there panting.

“Where did that noise come from?” Bob finally broke the silence.

“I don’t know,” Jupiter admitted reluctantly. “But I think we’ve done enough exploring for one night. Let’s start back.”

Bob and Pete were only too glad to follow the First Investigator back up the narrow trail. They had nearly reached the iron gate at the top of the cliff when Jupiter suddenly stopped. In the darkness Pete almost ran into him.

“What are you doing, Jupe!”

Jupiter didn’t reply. He was staring up at the twin peaks of Devil Mountain.

“What is it?” Bob asked in a whisper.

“I just had an idea,” Jupiter answered slowly. “And I also thought I saw something move up there on the mountain where — ”