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.

“Aaaaaaahhhhhhhhhhhh — oooooooooooooooo — oooooo — oo!”

The cave was moaning!

Jupiter was grinning like a contented cat. They were inside and the cave was actually moaning.

“Gosh, Jupe,” Pete whispered, “you were right! No one saw us come inside, so the cave’s moaning.”

“It certainly seems that way, doesn’t it?” Jupiter said somewhat smugly. “And it’s just about twilight — the exact time of our first visit here last night. Come on!”

Quickly they took off their diving gear. Jupiter struck a match from his waterproof box and lighted two of the candles.

“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.

“Aaaaaahhhhhh — ooooooooooo — ooooo — oo!”

It seemed to come straight out of the tunnel that was attracting the candle flame. Jupiter took out his chalk and marked a small white question mark at the entrance to the passage. Then the two boys lit their flashlights and plunged into the tunnel.

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.