“The boy’s right,” Old Ben said simply. “We knew it wasn’t a diamond mine. No diamonds around here.”

“We thought maybe it was a strike when we found the first couple, ”Waldo added, “only we really knew better, so Ben got those books. The diamonds turned out to be mostly African types. Then I went to the library and found a small piece in the local paper about that robbery. We got a copy of the San Francisco paper, and it described the stones, so we knew they were from the robbery.”

Old Ben took up the story. “The diamonds were stolen, so we figured we could keep them. No one except the crook was going to know. We started to dig and found a real bonanza!”

“Only the holes we opened up made the cave start to moan again,” Waldo went on. “At first we figured that was fine, it scared folks away from the cave. Then Mr. Dalton and the sheriff started looking around. So I went up on the mountain and any time anyone came near the cave I signalled Ben and he closed up the holes until they went away.”

Old Ben chuckled. “We sure had everyone fooled. I scared you boys off once myself, only I don’t figure how you got into the cave to-night without Waldo seeing you.”

Jupiter explained the ruse of Bob and the dummies, and the two old men listened with admiration. Old Ben chuckled when Jupe finished.

“By jinkers, I said you boys was smart, yessir. You had it figured, and you foxed us pretty good.”

Reston spoke sternly. “This is not a laughing matter, Mr. Jackson. Keeping stolen property is a serious crime.”

Ben grinned sheepishly. “I don’t know if we’d really have kept them. Only we’d never made a real strike, and it was kind of exciting to dig them up. For a while we felt like real prospectors again. I guess it wasn’t right, only we figured no one would be hurt except the thief. At least not until we decided what to do with the stones.”

“What about those accidents?” Bob asked hotly. “And the rock that almost hit us?”

“Most of them were real accidents,” Waldo explained. “They happen around here all the time. People got nervous from the moaning and that made them more careless. The one that almost hit you was my fault, though. I was watching you and my foot knocked over a stone and the boulder fell. I never meant to hurt anyone.”

Sam Reston looked at the two men severely. “I’ll decide what to do about you two later,” he said, gathering up the diamonds and putting them back into the leather bag. The two old men watched wistfully as their rich strike vanished.

“You’ve acted foolishly,” added Reston, “but you did recover the diamonds. Maybe you meant to return them eventually, who knows? Just now I have a thief to find.”

Jupiter spoke up again. “I’ve been thinking about Schmidt, Mr. Reston. I’m positive he knows Old Ben and Waldo have been digging in the cave, and he must know they have found his diamonds. I’m sure he will be back to get them, which leads me to think you set a trap for him.”

The muffled voice spoke from close behind them all.

“You are a smart boy. I did return!”

Everyone jumped, and turned towards the voice. There in the doorway stood the fake El Diablo! His masked face was as young and rigid as when he had captured Jupiter and Pete in the cave, and his left hand held the same pistol aimed at them all.

“Don’t move, boys,” Reston said quietly. “If this is Schmidt, he is a dangerous man.” The detective was eyeing his own pistol, which he had left on the table.

“Very wise advice,” the muffled voice rasped. “And it is indeed Schmidt.” The thief waved his pistol to indicate they were to move against the wall. “Don’t try for that pistol, Reston.”

Reston, the boys, and the two old men stood against the wall.

“You, the small boy, take that rope in the corner and tie up Reston. Quick!”

“Do it, Bob,” Reston said.

Swallowing hard, Bob got the rope and tied Reston’s hands and feet. Schmidt motioned him away and inspected Reston’s bonds. Satisfied, the bandit stepped back.

“Now you boys tie the old men,” the bandit ordered.

Jupiter and Bob tied Old Ben and Waldo. Then Bob tied Jupiter, and Schmidt himself tied up Bob. When they were all trussed up on the floor, the bandit stepped to the table and picked up the leather bag. His voice rasped mockingly.

“I must thank you for having the diamonds ready for me. You saved me all the trouble of digging them out after the earthquake. I’ve been watching them quite carefully, of course. I did not go to the trouble of stealing them to lose them that easily.” The bandit chuckled. “You boys were a bit stubborn and troublesome, but when I saw those scuba tanks I guessed what you were up to. I was a trifle nervous when I realized Reston was close behind me again, but it has all turned out fine.”

The jewel thief bowed mockingly to his trussed victims, and vanished from the cabin.

Jupiter groaned. “I should have guessed he would be watching us! When he captured us in the cave it was obvious he knew about the digging — we could hear it from where he caught us.”

“Don’t blame yourself, Jupiter,” Reston said. “You solved the entire case correctly. I should have realized myself that Schmidt was only using Old Ben and Waldo.”

“Well,” Bob said, “at least Jupe guessed right. The thief did return.”

Jupiter frowned, unsatisfied. “What good is solving a mystery if you can’t even see the villain’s face?” he asked. “He’ll get away, and we’ll never know what he looks like. And Mr. Reston will have to start all over — ”

Jupiter stopped in mid-sentence, his mouth open like a startled fish. He sat staring ahead as if in some kind of trance.

“Jupe?” Bob said.

“Jupiter,” Sam Reston said, “what is it?”

Jupiter blinked as if he had just come back into the room from a long trip. “We’ve got to get loose!” he cried, straining at his bonds. “We’ve got to hurry and go after him!”

Sam Reston shook his head gloomily. “He’s long gone by now, Jupiter. He wouldn’t stay around.”

“I don’t know,” Jupiter said.

“You don’t know what, Jupe?” asked Bob.

The sudden sound of horses’ hoofs outside the cabin prevented Jupiter’s reply. A moment later the outside door burst open, and a big man they had never seen before glared down at the five bound prisoners.

“What the devil is going on in here?” he boomed. “You boys should know better.”

Bob and Jupe looked up at the big man, and then grinned with relief.

Behind him they saw the familiar and friendly faces of Pete and Mrs. Dalton.

The big man proved to be the sheriff of Santa Carla County, and at first he was very angry with the boys for trying to solve the mystery alone.

“Three boys have no business chasing a dangerous jewel thief!” the sheriff thundered.

“Anything might have happened in that cave,” added Mrs. Dalton, “with all kinds of thieves and crazy men walking around! If Pete hadn’t spotted those question marks and realized you might have gone to Old Ben’s cabin, goodness knows how we would have found you!”

Bob looked a little bit sheepish, but Jupiter turned quickly to the sheriff. “We’re sorry, sir,” he said politely, “but we did nothing really dangerous in the cave. We were most unfortunately captured by the thief Mr. Reston is pursuing.”

Reston broke in. “That’s right, Sheriff. The boys had no way of knowing that there was a dangerous criminal in the cave. They thought they were merely solving the mystery of the moaning, and perhaps of a couple of eccentric but harmless old men. They had no idea of capturing a jewel thief until I came along. It was my idea to go after Old Ben and Waldo.”

“And that’s something I want to talk to you about later,” the sheriff growled at Reston. “But maybe you’re right. I guess the boys acted pretty responsibly all in all.”