“Of course the top of the pass is only a few miles from here,” Jupiter mused. “Anyone from the ranch or the valley could have driven up there in just a few minutes and probably not have been missed.”
“Gee, I guess so,” Bob agreed.
“Still,” Jupiter went on thoughtfully, “a Nevada licence plate is most interesting. As far as we know, no one around the ranch has anything but a California plate.”
“You mean there’s someone around here we don’t know?” Pete said.
“Sure there is,” Bob told him. “That man with the eye patch.”
“It looks that way,” Jupiter agreed, “but now we must get to work. I’ll look through that book on Moaning Valley while you two go down and check the scuba equipment. Wrap the tanks in something that will disguise them, and then put them on the bikes with the candles, the sombreros and the bundle I brought.”
“The plan!” Bob and Pete said together. “What is it?”
“I’ll tell you on the way,” Jupiter said, and looked at his prized chronometer. “We have to hurry now if we’re going to reach Moaning Valley before sundown. To-night we may solve the mystery of Moaning Valley!”
Half an hour later the First Investigator appeared in the barn, waving the book at Bob and Pete.
“I think I’ve found part of the answer,” Jupiter announced. “It says here that about fifty years ago they sealed up many of the old mine shafts in Devil Mountain. They had never found any gold or anything else, so they closed the tunnels. Fifty years ago was when the original moaning sound stopped!”
“You mean one of them has been opened up again?” asked Bob. “And the wind blowing through it causes the moaning sound?”
“Yes, I think so,” Jupiter agreed. “The question is how and why?… Are you fellows ready?”
“Ready, Jupe,” Pete said.
“All right, then put on the sombreros before we ride out of the barn,” Jupiter instructed.
The boys donned the wide-brimmed straw hats, balanced the heavy tanks disguised in burlap sacks, and mounted their bikes. The bikes proved somewhat hard to handle with the weight on them, and it was clear that they would have to pedal with care.
“Ohhh,” Bob cried, wincing with pain.
“Is it your ankle, Bob?” asked Pete.
“It’s all the weight on the bike,” Jupiter decided.
Bob nodded unhappily. “I don’t think I can make it, Jupe. I guess I’ll have to stay behind.”
Jupiter had his thoughtful look. “No, I don’t think you will have to stay behind, Bob. Perhaps we can turn this misfortune into an advantage. It will make our deception more convincing.”
“What deception?” Pete asked, bewildered.
“The classic military tactic of the camp-fires and logs that look like cannon,” Jupiter explained obscurely. “Bob, unload your scuba equipment. Without the weight I think you can operate the bike.”
Bob tried again and found that without the extra weight he could indeed pedal satisfactorily. The boys rode out of the barn towards the gate. As they passed the house Mrs. Dalton waved from the porch.
“Have a good time, boys, and don’t stay out too late!” she called. “And be careful!”
Once out of sight of the ranch, the boys pedalled faster towards Moaning Valley. When they reached the place where the road ended at the iron gate, they dismounted and carried their bundles and their bikes into the thick bushes.
“Now,” Jupiter said, “here is my plan. We’re going to get inside the cave without being seen.”
Pete nodded. “I get it. We’ll take the moaning by surprise.”
“Right,” said Jupe. “Of course, if my theory is correct we’re being watched closely right now!”
“Gosh,” Bob said, “then how do we do it?”
“We go under water,” Jupiter told them, “using the scuba equipment. I checked on the tide and it’s higher this evening. I estimate that most of the tunnel from the beach will be under water.”
“But, Jupe,” Bob objected, “how will we get into the water without being seen if we’re being watched now?”
Jupiter beamed triumphantly. “We will use the decoy tactic. The way armies used to light camp-fires at night, and then slip away in the dark.”
“But — ” Pete began.
“You see,” Jupiter went on, “I also noticed last night that while the trail to the right is in clear view from the top of Devil Mountain, the trail to the left is hidden. Come on. Walk casually and in the open.”
The three boys climbed over the iron gate and continued down the cliff path to the left. When they were just out of sight from the top of Devil Mountain, Jupiter said, “Stop here.” The boys put down the scuba tanks, and watched as Jupiter opened the secret package.
“That’s just old clothes!” exclaimed Pete.
“The same as those we’re wearing!” Bob added.
“Exactly,” said Jupiter. “Stuff them with brush, and tie off the arms and legs with this cord.”
Bob and Pete did as Jupe said and in a few moments they had two dummies that looked remarkably like Pete and Jupiter.
“And the sombreros will hide our faces!” Pete said.
“Precisely,” agreed Jupiter. “In addition they can be easily seen from the top of the mountain. Whoever is up there will be convinced that the dummies are us, especially since Bob will stay here with them and move from time to time!”
Quickly the boys set up the dummies above the trail. Bob sat beside the dummies, pretending to talk to them. From a distance it would look exactly as if The Three Investigators were sitting at the cliff edge observing the view.
Hidden below the cliff, Jupiter and Pete slipped down the trail to the small beach at the bottom. There they struggled into their air-tanks.
“The surf is light to-night,” Jupiter said. “We should have no trouble swimming from here to the cave entrance.”
Pete nodded. “Underwater it shouldn’t take more than five minutes to swim that far with our nippers.”
“Right,” Jupiter agreed. “I have my compass and if necessary we can surface briefly. Our decoys should keep anyone from bothering to watch the ocean.”
The boys fixed their breathing tubes in their mouths, walked backwards into the water, and slid beneath the waves.
Fish darted around them, and one large halibut, which had, been invisible against the bottom, startled Pete by separating itself from the rocks and swimming majestically away.
After a couple of minutes, Jupiter stopped and turned to face Pete. He pointed to his diving chronometer and then towards the shore. Pete nodded. It was time to start in for El Diablo’s Cave.
Jupiter continued to lead the way. Closer to shore the water was murky, and there were more rocks, so Pete swam closer to the flapping fins in front of him. In fact, he swam so close that he bumped hard into Jupiter’s back when the First Investigator suddenly stopped.
Pete grunted, a little annoyed. His annoyance abruptly vanished as he saw that Jupiter was frantically pointing to the left. Pete looked.
A dark shape moved slowly through the water no more than thirty feet from them. It was large and long, like a big, black, hazy cigar — the shape of a shark or even a killer whale!
Pete’s heart thudded. But the boys had been carefully instructed about what to do if they ran into a shark. They reacted to their training instantly. Moving as little as possible, since movement would attract a shark, they sank to the bottom. They drew their diving knives as a precaution, and began to edge their way slowly towards the safety of the rocks.
Pete watched the shape intently. He decided that it moved too steadily, too rigidly in a straight line, and was too long for a shark. At the same time, it seemed too small and slow for a killer whale.
Jupiter touched him on the shoulder and made the sign for a shark. Pete shook his head and both boys watched the strange shape slowly fade away out to sea. Then they swam in until the surge of the surf told them they were close to the cliff of Devil Mountain. They surfaced cautiously and found themselves only a few feet from the cave mouth.
“What was it?” Jupiter asked as soon as he removed his mouthpiece.
“I don’t know,” Pete replied nervously. “I’m pretty sure it wasn’t a shark or whale or any kind of fish. Maybe we should go back, Jupe, and get the sheriff.”
“If a whole posse came here they wouldn’t find anything,” Jupiter pointed out. “Whatever that shape was, it was going away, right? I’m sure that there is some simple explanation for it and, anyway, it’s gone now.”
“Well… ” Pete hesitated.
“Now that we’re this far, it would be foolish to turn back without investigating the moaning,” Jupiter said decisively. He always hated to give up once he was on a trail.
“Come on, Pete. I’m going into the cave. You hold the rope until I’m inside.”
Jupiter vanished under the water. The sun was almost down now, and in the darkening twilight Pete waited with the rope in his hands. When he felt a double tug, he fixed his mouthpiece in place and swam into the narrow passage.
There was little surf and no current and Pete’s waterproof flashlight, which was fastened to his gear, gave plenty of illumination. The water in the tunnel became quickly shallow as the floor angled upward, and soon Pete was standing in the large cavern beside his friend. The first thing he noticed as he took off his swim fins was the sound.
“We’ll carry the candles to the mouths of all the tunnels that lead out of this cavern,” Jupiter explained. “If the candle flickers it means there is a current of air moving through the tunnel. If the flame doesn’t move, that means the passage is probably blocked. It will save us a great deal of time and wasted searching.”
Pete nodded. “Smart idea!”
Quickly they began to test the passages. At one, the candle flickered slightly. But Jupe was not satisfied. Pete went into the next tunnel. Suddenly the flame of his candle was drawn strongly into the dark opening.
“Here, Jupe!” Pete called excitedly.
“Shhhhhh!” whispered Jupiter. “We don’t know how close we may be to somebody.”
Both boys held their breath and listened. For a long half-minute all was silent and Pete was furious at himself for shouting. Then the moan came again, faint but clear.
Back at the cliff top, Bob sat with the dummies, watching the flaming orange sun set in the west. Slowly, a purple and red twilight settled over the ocean. Bob stretched his legs cautiously.
He had been sitting there talking to himself for over a half-hour, he guessed, and during all that time he had had the feeling that eyes were watching him. He knew that it was probably just his imagination, but it was a weird feeling anyway.
To occupy his mind, Bob began to read the book about Moaning Valley. He read the part that told of the mine shafts being sealed, and then he read further. Suddenly he sat up very straight.
“Gosh!” he exclaimed in a whisper.
He had come to a passage about Old Ben Jackson and his partner, Waldo Turner. The book said that Old Ben and his partner lived on a ridge right next to Devil Mountain and that they had dug one of the mine shafts into the mountain. Their shaft had been sealed up with the others, but Old Ben and Waldo had refused to leave. They insisted they would keep prospecting for gold — and diamonds!
Bob frowned. He was sure that Jupiter, in his eagerness to get started on his plan, had not read this far in the book. If Jupe had read that Old Ben thought there were diamonds around Devil Mountain, he would have mentioned it.
In the growing darkness Bob was suddenly worried. Jupiter thought that the moaning might be caused by the reopening of one of the old shafts. Old Ben and his partner had actually dug such a shaft themselves, and they probably knew El Diablo’s Cave better than anyone else, after living right next to it for so many years. It would be simple for them to reopen a shaft.
Then Bob thought about something else. He remembered how Old Ben had surprised them the previous night. They had been inside an inner cavern, yet Old Ben had claimed he was passing by outside the cave and heard them! Suddenly, Bob realized that this would have been practically impossible. The distance was too great. Old Ben must have been inside the cave when he heard them, which meant that he had lied to them.
Alarmed now, Bob dropped below the trail and hurriedly made a third dummy from the shirt, pants and sombrero that had originally been intended to represent him. Carefully, he pushed it into place beside the other two. In the dim twilight the three dummies should convince any observer that the boys were still seated there.
Then he crept through the underbush until he felt it was safe to stand up and walk. He kept a good distance from the road, for he did not want to be seen. He felt it was very important that he get back to tell the Daltons what Pete and Jupe were doing in the cave. If Old Ben had actually found a diamond mine, they could be in real danger!
Bob hurried through the night as fast as he could with his injured leg and the difficult terrain. Before he had gone more than a few hundred yards he heard a soft sound in the night. It was a car driving slowly along the dirt road — without lights! It stopped no more than fifty feet from where Bob crouched.
A shadowy figure got out and walked rapidly towards Devil Mountain. The dark shape was dressed completely in black, and was all but invisible in the night. It quickly vanished.
Bob crept up to the parked car. It had a Nevada licence plate.
Deep inside Devil Mountain, Pete and Jupe continued to track the moaning sound. After the first tunnel, they had come to another cavern and had again used their candles to locate the passage out. In the third cavern, smaller than any of the others, they had found three passages with air blowing through them. They decided not to split up. Instead, they searched each passage together.
The first tunnel ran straight ahead for quite a distance, then made a sudden, sharp turn.
“It’s heading back towards the ocean, Jupe,” Pete observed.
Jupiter frowned. “I don’t think we want to go that way. I’m sure the moaning sound comes from closer to the valley side.” He checked his compass. “We should go east or north-east, I think.”
“This tunnel is heading south-west.”
The boys retraced their steps and tried the second passage. Soon it, too, curved away towards the southwest. Once again they went back to the cavern. Pete was becoming impatient.