“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.

Pete followed Jupiter’s waving fins through the bright, translucent water. Both boys were experienced scuba divers and swam only with their feet, with no wasted motion. Pete maintained a wary watch on the dark shadows of the rocks, while Jupiter concentrated on following the direction with his wrist compass.

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.