The sound of jingling carried across the night along with the clip-clop, clip-clop, clip-clop of a horse’s hoofs.

“Oh no!” Bob groaned.

“Is that what we heard inside the cave?” whispered Pete.

“I think so,” Jupiter said. “The sound must have filtered down through some crevice in the rock of the mountain. Sound carries very clearly that way. It would have seemed as if it was inside the mountain itself.”

The boys crouched down in the thick bushes near the gate as the hoof beats came closer. Then a great black horse appeared on the steep slopes of Devil Mountain. It came down the mountain at a trot and passed within a few feet from where the boys crouched in the bushes.

“There’s no rider!” Bob whispered.

“Should we try to catch it?” asked Pete.

“No, I don’t think so,” Jupiter replied. “Let’s wait.”

The boys crouched silently under cover of the bushes. Suddenly Pete stiffened, and pointed towards the slope. A man was coming down the mountain at a fast walk. As he passed their hiding place, they had a clear view of him in the moonlight. He was a tall, dark man with a long nose, a ragged scar on his right cheek, and a black patch over his right eye!

“Did you see that eye patch?” hissed Pete.

“And the scar,” Bob added.

“I was more interested in his suit,” Jupiter whispered. “It was definitely a business suit, and unless I am mistaken I think he had a pistol under his coat!”

“Can’t we go now, Jupe?” said Pete nervously.

“Yes, I think we’d better,” Jupiter agreed. “This has been a most interesting night.”

Pete and Bob needed no urging. As they hurried along the road to where they had left their bikes, they kept looking behind them anxiously. But they saw nothing else. While they were riding past the end of Moaning Valley, however, a long wail echoed through the night.

“Aaaaaaaaaaaahhhhhhhhh — ooooooooooooooo — ooooo — ooo!”

The boys began to pedal furiously in the direction of the ranch house.

Pete woke up in bright sunlight. Confused, he looked around the unfamiliar room. Where was he? Then a horse whinnied somewhere outside, a cow lowed, and Pete remembered that he was in the upstairs bedroom of The Crooked-Y. He leaned over the edge of his top bunk to see what Jupiter was up to. Jupiter was not there.

Pete sat up quickly, bumping his head on the low ceiling.

“Ouch!” he grunted.

“Shhhhhh!” Bob hissed from his bunk across the room, and pointed towards the window.

In front of the window Jupiter sat cross-legged on the floor, looking like a small Buddha in his bathrobe. A large sheet of paper was spread out on the floor in front of him and in the middle of it he had stacked four books. On the paper Jupiter had drawn a lot of pencil lines.

As Pete stared down at the books and the paper and the pencil marks he realized that Jupiter had made a rough model of Moaning Valley. He had marked the cave entrances in pencil.

“He’s been sitting like that for an hour,” Bob explained.

“Gosh,” Pete said. “I couldn’t sit like that for ten minutes!” The intense concentration of their stocky friend always awed Bob and Pete.

Suddenly, Jupiter spoke. “I am ascertaining the exact topographical arrangement of Moaning Valley, Pete. The key to our puzzle lies in the physical pattern.”

“Huh?” Pete said.

“Jupe means that he thinks the mystery can be solved by studying the lay of the land,” Bob explained.

“Oh,” Pete said. “Why didn’t he say so?”

Ignoring Pete, Jupiter went on, “The real mystery of Moaning Valley is why does the moaning stop when we go inside? It happened twice last night, yet when we were leaving the area the moaning began again.”

He held up a newspaper. “I have here a newspaper report of the sudden recurrence of the moaning. In it the sheriff says that the main reason no one has been able to identify the cause of the moaning is that once inside they never hear it any more.”

Jupiter put down the paper. “I’m convinced now that the moaning doesn’t stop by accident!”

“I guess you’re right,” Bob said. “The way it started again right when we were leaving sure looks like someone was watching us.”

“But how does that — uh — model help us, Jupe?” Pete blurted out.

Jupiter looked down at his crude model. “I’ve marked all the places we were last night. Now we know that both times we entered the cave the moaning stopped instantly. It happened too quickly for it to have been someone inside the cave who was watching us.”

Bob nodded eagerly. “I get it! So we had to be seen before we went in.”

“Exactly,” Jupiter said, “and from my model I’ve deduced that we could have been seen everywhere we went from only one place — the top of Devil Mountain.”

“Then all we have to do is tell Mr. Dalton someone is up on Devil Mountain and let him catch the person!” exclaimed Pete.

Jupiter shook his head. “No, Pete, no one would believe us unless they caught the man and it would be almost impossible to get to the top without being seen. Whoever is up there would just run away.”

“Then — ” Bob began.

“How — ” Pete started at the same instant.

“We will have to observe what is actually happening in the cave,” Jupiter said solemnly, “so that we can tell people the full story.”

“But we don’t know what’s happening in the cave,” Pete objected. “Do we?”

“No, but I’ve got a plan in mind,” revealed Jupiter, “and I’ve got a clue as to what it’s all about!”

“You do?” Pete said. “What is it?”

“Last night I found this in one of the passages,” Jupiter explained, holding up the rough, blackish stone he had found in the mine-shaft passage. “The passage was once a mine shaft, and I found this stone just where it ended in one of those blockages.”

Bob took the little stone, looked at it with a puzzled expression and passed it on to Pete.

“But what is it, Jupe?” Pete demanded. “I mean, besides a kind of hard, slippery stone?”

“Scratch the window with it,” Jupiter said.

“What?” Pete said, surprised. “You know it won’t — ”

“Go ahead,” Jupiter urged, a smug expression on his round face.

Pete went to the window and scratched the small stone over the glass. It cut into glass almost as easily as a knife cutting into butter. Pete let out a low whistle.

“Jupe!” Bob exclaimed. “You mean that’s a — ”

“Diamond,” Jupiter finished. “Yes, I think that’s exactly what it is. A rough, uncut diamond. And a pretty big one. I think its quality isn’t good, probably no more than an industrial stone. But it is a diamond.”

“You mean El Diablo’s Cave is a diamond mine? Here in California?” Bob asked sceptically.

“Well, there have been rumours, I think, and — ”

That was as far as he got. A vigorous knock on the bedroom door interrupted him, and Mrs. Dalton’s voice called, “Up and at ’em, boys! Breakfast’s on the table. We’ll have no late sleeping here!”

Everything else was forgotten for the moment as the boys realized how hungry they were. They dressed and were in the big ranch kitchen five minutes later. Mr. Dalton and Professor Walsh smiled at them.

“Well, I see that Moaning Valley and its mystery have not affected your appetites, boys,” Professor Walsh commented.

Mrs. Dalton bustled around the roomy, bright kitchen and soon the boys were digging into stacks of buckwheat cakes and ham, and drinking mugs of cold fresh milk.

“You boys ready to do some work today?” Mr. Dalton asked.

“Of course they are,” Mrs. Dalton said. “Why not take them up to the north meadow for the haying?”

“Good idea,” Mr. Dalton agreed. “Later they can help round up some mavericks.”

The boys, who had done some reading on ranch life, knew that “mavericks” were cattle that had strayed from the main herd into remote parts of the range.