“Maybe he ate and drank somewhere else,” Pete suggested.

“Perhaps, but then what killed him? If he was in good health and was attacked there should be signs of a fight, and perhaps another skeleton or two. Also, if anyone had found El Diablo in the cave and killed him, surely the historical record would show that.”

“Gee, I guess you’re right,” Pete agreed.

“In addition,” Jupiter went on, “note the position of the skeleton. He literally died with his back to the wall. He was seated here ready to fight if any enemy appeared, but I don’t think an enemy ever came. Look at the pistol.”

Pete picked up the pistol. “It’s full, Jupe. No shots fired.”

“Just as I thought,” Jupiter said in triumph. “His place of hiding was never discovered and he died alone in here of his wounds, just as the historical record states. Everything is consistent with that conclusion. El Diablo did know the cave better than anyone else.”

“Maybe it would have been better for him if he hadn’t known it so well,” said Pete. “I mean, maybe if they’d found him they could have taken care of his wound.”

“Perhaps, but you’ll recall he was under sentence to hang anyway. I imagine he preferred dying in his cave to being recaptured. He may even have guessed that if he was never found his legend would grow, and perhaps in some way help his people.”

“It sure grew,” said Pete.

Jupiter nodded. “So much so that someone is now using it to scare us — and anyone else who comes into the cave. The question is why?”

“Maybe someone wants to make Mr. and Mrs. Dalton lose their ranch,” Pete suggested.

“That’s possible,” Jupiter conceded, “but I don’t think so. I think someone is trying to scare people away from the cave. Remember that the Daltons have been here for some time, but the moaning only started a month ago.”

“Golly, Jupe, if someone is trying to scare people away, how come no one saw the phoney El Diablo until to-night? I mean, why didn’t he appear when the sheriff and Mr. Dalton explored the cave?”

“I don’t know that yet,” Jupiter admitted. “But until to-night the moaning always stopped when anyone entered the cave. To-night we managed to enter unseen, the moaning did not stop, and the fake El Diablo appeared! This leads me to the deduction that we saw El Diablo to-night because the moaning had not stopped.”

“That makes less sense than anything else,” Pete protested. “What do you think it means?”

For once Jupiter looked completely baffled. “I don’t know, Pete. But I do know that there is more to the mystery of Moaning Valley than some natural cause for the moaning. We have to find out what that digging we heard earlier was all about.”

“Gosh, I’d forgotten all about the digging. Do you really think there’s a diamond mine in the cave?”

“I think someone is trying to conceal something,” Jupiter explained. “Last night I found a diamond. To-night we heard somebody digging. Logic indicates that a diamond mine must be involved somehow.”

“Maybe we should tell Mr. Dalton what we know, Jupe,” Pete said uneasily.

Jupiter frowned. The First Investigator hated to admit that they could not handle a situation on their own, but even he had to agree that there were times when three boys could not do the job alone.

“I suppose you’re right,” Jupiter said reluctantly. “Bring El Diablo’s pistol and we’ll try to find the tunnel that leads out of here.”

Pete lit his candle, and the boys started for the next tunnel to test for a current of air.

Suddenly there was a faint ripple in the water of the pool that had been so dark and silent. This was followed by splashing and loud breathing. The boys stood motionless, their flashlights trained on the sound.

A black and shiny shape broke the surface of the murky pool. Water dripped from its shining skin, reflecting in the beams of the boys’ flashlights, as the creature raised itself out of the water.

Jupiter and Pete stared in horror as the shiny black creature began to climb out of the pool.

“What are you boys doing here?” the creature demanded.

Suddenly, at the same time, both boys realized what they were looking at. It was a man wearing a black rubber wet-suit, swim fins, a double air-tank painted black, and a black rubber mask that completely covered his face.

“Oh, boy!” Pete exploded in relief.

Jupiter, recovering immediately, drew himself up to his full height, his round face suddenly looking much older. It was an old trick he used when dealing with adults, and it usually worked as well as his other surprises.

“What, sir, are you doing here?” he asked in his deepest voice. “We are here by permission of the owners of this ranch. You, apparently, have come in by some secret entrance that leads from the sea. You are trespassing.”

The skin-diver reached up and removed his rubber face mask. He was a handsome, blond man and he was grinning at Jupiter. Unhooking his air-tanks, he laid them on the floor.

“Well, son, you sound almost as important as the admiral,” he said. “I wasn’t questioning your right to be here. I was just wondering what two boys were doing in El Diablo’s Cave this late at night.”

“The admiral?” Jupiter looked puzzled for a moment. “Of course! You’re a frogman, aren’t you? A Navy frogman on training manoeuvres with those ships out near the islands.”

The frogman looked serious. “Yes, that’s exactly what I am. We’re on a highly secret training mission here. I’ll have to swear you boys to complete secrecy. Have you seen anything in the water you thought was unusual?”

“No,” Pete said.

“Nothing, sir,” Jupiter assured him. Then, remembering suddenly, he snapped his fingers. “Except that shape!”

“Shape?” the frogman repeated.

Now Pete remembered too. “That long, black thing that passed us out in the ocean.”

“It was a submarine, Pete!” Jupiter exclaimed eagerly. “A midget submarine. That was why it was so rigid, and why it moved so steadily. But why didn’t we hear its engines? Sound carries very far under water.”

The frogman’s face darkened. “This could be very serious, boys. That submarine you saw is top secret — especially the silence of its engines. I’m afraid I’ll have to hold you.”

“Hold us?” Pete echoed.

“A submarine that moves so silently that it can’t be detected by sonar is very important, Pete,” Jupiter said solemnly. “However, I believe we can prove that there is no need to hold us, Mr…. ”

“Commander Crane,” the frogman said. “Commander Paul Crane. And I’m sorry, but I am going to have to hold you — at least until the admiral can have you investigated.”

Jupiter nodded understandingly and tried to look dignified, which wasn’t easy when he was wearing only bathing trunks and a diving belt.

“We’re Jupiter Jones and Pete Crenshaw,” Jupiter said, and reached into one of the waterproof containers which hung from his diving belt. “I believe these credentials will attest to our complete reliability.”

Jupiter handed the commander the boys’ business card and the special deputy cards given to them by Chief Reynolds of the Rocky Beach police. Commander Crane studied the cards.

“We happen to be involved in an important case right now,” Jupiter told him. “That is why we are in this cave. I’m sure your admiral would want you to co-operate with us. Commander.”

Commander Crane looked at Jupiter and hesitated. The First Investigator could be very impressive when he was being serious and professional.

“Well now,” the frogman said, “these cards do make you boys seem legitimate.”

“Why don’t you communicate with your ship,” Jupiter suggested, “and have them check at once with Chief Reynolds in Rocky Beach. I’m sure he will vouch for us.”

“Gosh, Jupe,” Pete exclaimed, “how can the commander talk to his ship from in here?”