“How come the sheriff and Mr. Dalton don’t know it’s there?” Pete asked.

“It’s probably concealed,” guessed Jupiter. “There has to be another entrance up high on the mountain, too, for Waldo to get down here so fast. There are probably quite a few hidden entrances. However, I think it’s time we went for help.”

“Let’s go!” Pete agreed fervently.

The boys switched on their flashlights and walked rapidly back along the mine shaft. By retracing their steps, they soon reached the first large cavern they had entered the previous night.

As they hurried towards the tunnel that would lead them outside, two figures leaped out of the shadows. Strong hands closed on Pete’s arm.

“Got you!” a harsh voice grunted.

Pete gulped in fright as his light shone up into the long, scarred face of the man with the eye patch.

“Run, Jupe!” Pete cried.

At the same instant, a flashlight was suddenly beamed on Jupiter by the second man.

“Just stay where you are,” the man with the eye patch commanded. “If you start running around in the dark you’ll get hurt.”

Jupiter braved it out. “I doubt that you would care if I were hurt. I suggest you let us go. We have friends here.”

The man laughed. “Spunky, aren’t you?… Why don’t you come over here where we can have a talk.”

“Don’t, Jupe!” Pete cried.

And then a familiar voice spoke from behind the flashlight of the second man.

“It’s okay, fellows. Mr. Reston is a detective!”

The voice was Bob’s and his face was one big grin when he came out into the light and saw the astonished looks of his partners.

“I started for the ranch to get help after I saw that the man in the Nevada car was going into the cave,” Bob explained. He went on to describe his sudden hunch that Old Ben and Waldo were involved in the mystery of Moaning Valley. “After the Nevada car passed me I was scared and ran right into Mr. Reston here.”

“Sam Reston,” the man with the eye patch, introduced himself. “I’m a detective, boys, working for an insurance company. When your friend here told me of his suspicions about Old Ben, I decided to come back to the cave with him instead of going all the way to the ranch for help.”

“Mr. Reston thought you might need help right away,” Bob explained.

“I did,” Reston said, “because the man I’m after is very dangerous. Bob and I tried to get into the cave unseen. It took us some time, and I think we were seen anyway.”

“You were, Mr. Reston,” Jupiter said, suddenly recovering his voice. The First Investigator told Bob and Mr. Reston about everything he and Pete had seen in the cave.

Reston nodded. “I was afraid we’d been spotted. But they can’t have gone far, and that bag you saw probably contains the diamonds I’m after.”

“What diamonds?” Pete burst out.

“That’s the job I’m on, boys,” Reston explained. “I’m trying to find a very clever jewel thief who stole a fortune in diamonds. His name is Laslo Schmidt, and he is known all over Europe. I followed his trail here to Santa Carla just a week ago. Then I heard about Moaning Valley and El Diablo’s Cave, and I had the idea that the cave would be a good place for Schmidt to hide. Only I haven’t found a trace of him.”

“Gosh,” Pete said, “if you followed his trail here, why couldn’t you spot him?”

“Because I have no idea what he looks like now,” Reston told them. “You see, boys, about five years ago Schmidt left Europe in a hurry. The International Police, Interpol, learned that he had come to America and assumed a new identity. But that was all they could find out. Schmidt is a master of disguise and impersonation. He could act the part of almost anyone and make you believe it.”

Jupiter had his faraway thoughtful expression. “And he stole some diamonds insured by your company, Mr. Reston?”

“Yes, about a year ago. He hadn’t stolen anything since he left Europe, and the police thought that he had given up, or perhaps even died. But when the diamonds were stolen we knew at once that Schmidt was the thief. The way it was done, it could have been no one else.”

“The modus operandi, or method of operation is very important,” agreed Jupiter. “That is how most criminals are caught, especially professional thieves. A thief can never really change his way of committing a theft except in minor details.”

“That’s right, Jupiter,” Reston acknowledged. “The theft was obviously the work of Laslo Schmidt, and we realized that he had been simply biding his time. It became clear that he had spent his years in this country establishing a new identity. So now he is actually two men: Schmidt the thief, and some other person who is perfectly normal and above suspicion.”

“And you don’t know what this other identity is,” Bob put in quickly. “It could be anyone around here.”

Reston nodded. “Exactly, Bob. I traced him through the sale of two of the diamonds. First to Reno, Nevada, and then here.”

“Nevada!” Pete and Bob both exclaimed at once.

“Gosh,” Pete added, “we thought you were driving that Nevada car that forced us over the cliff.”

“No, boys,” Reston explained. “I was on my way to Moaning Valley when I saw your bikes at the edge of the pass. I stopped to see what was wrong, and would have rescued you, but then I saw those other people coming and knew you’d be okay. At that time I didn’t want to reveal my presence here. You see, I think Schmidt spotted me in Nevada. I tried to fool him by putting on this eye patch and pasting a false scar on my face before coming to Santa Carla. I’m not sure my disguise was successful, though.”

“That’s why you’ve been sort of hiding?” Bob asked.

“That’s right. I didn’t want Schmidt to know I was still on his trail.”

While Reston had been talking, Jupiter had been staring off into the dark spaces of the cavern and biting his lip. Now an eager gleam came into his eyes.

“The diamonds he stole,” Jupiter said slowly. “There is something special about them, isn’t there, Mr. Reston?”

Reston looked at him in astonishment. “Why, yes, Jupiter, there is. You see, they weren’t stolen from any jewellery company or store. They were taken from a special display at a museum in San Francisco. They — ”

“—are rough diamonds!” Jupiter completed the sentence for him. “They are uncut, exactly as they came from the diamond mine, correct? They are also industrial diamonds.”

“I don’t understand how you know,” Reston said, “but you’re right about their being rough diamonds. Only a few are just industrial stones, though. You see, the display was made up of diamonds from all over the world, just as they came out of the ground. Because they look like ordinary rocks, and because they were in a museum, they were not very well-guarded. Schmidt had no trouble stealing them. Most of them are gemstones, very valuable and just about unidentifiable since they are rough. But how did you know, Jupiter?”

“Because I found a rough diamond here in the cave,” Jupiter said, “and because I think Old Ben and Waldo have found the rest of them!”

“Then the stones really are in the cave!” Reston exclaimed.

Jupiter nodded solemnly. “I think your Laslo Schmidt hid them here right after he stole them. He probably planned to keep them hidden until everyone gave up the chase. Only Old Ben and Waldo, prospecting secretly in the cave as they have probably been doing for years, found them and thought that they had uncovered a diamond mine!”

“But there are no diamond mines in this area,” Reston pointed out.

“No, sir, but Old Ben and Waldo have always believed that there are diamonds here. I remember Mr. Dalton said they have been looking for precious stones as well as gold and silver. The diamonds Schmidt stole look exactly as they would if they had just been dug up, don’t they?”