“A good frogman is always in contact with his ship,” Jupiter informed Pete. “I assume that the commander has some form of long-range radio.”

Commander Crane smiled. “You’re a very smart boy, I see. All right, you boys just sit down and stay put.”

Jupiter and Pete did as they were told, and Commander Crane vanished into the darkness. Minutes passed. The boys could barely see the frogman crouched in the dark of the cavern. He was bending over a tiny instrument of a type the boys had never seen before. Jupiter watched with curiosity, but he could not make out what was happening.

At last the commander straightened up again, replaced his instrument in a hidden pocket, and strode back to the boys. He was smiling.

“Security says you boys check out,” he said. “I won’t have to hold you.”

“Gosh, you move fast,” Pete said.

“We move fast when we have to,” replied Commander Crane. “The admiral has pretty high priorities.”

“Now that we’ve been approved, Commander,” Jupiter said seriously, “may we ask you some questions?”

“Me?” The frogman shook his head, smiling. “I’m afraid that won’t be possible, boys. My work is also highly secret.”

“It’s not about your work exactly, sir,” Jupiter assured him. “I want to know about this cave. First, was it you Pete saw last night up near the front of the cave?”

Commander Crane nodded. “It was probably one of my men. He reported that he had been spotted briefly.”

“That makes me feel better,” Pete said. “At least it explains another mystery of the cave.”

“Second,” Jupiter went on, “did you or your men make any changes inside, the cave? I mean, did you change the layout of the cave, the various tunnels and openings, or anything like that?”

“No,” the commander said, “I can tell you that much.”

“Third, sir,” Jupiter enumerated, “is anything you are doing causing the cave to moan as it does?”

“Absolutely not. We wondered about that moaning ourselves. Of course, we’ve only been in the cave a few times. We haven’t been in this area long. We assumed the cave always moaned like that.”

“And your work requires you to remain completely unseen if possible?” Jupiter pressed on.

“Absolutely.” The commander smiled. “As a matter of fact, I’m sure we haven’t been seen by anyone but you boys. Most of our work has been done on the ocean side of the cave, and here near this pool.”

“Have you seen anyone else in the cave?” Jupiter asked.

Commander Crane shook his head. “No, it is essential to our training mission that we remain unseen. Of course, there is no enemy here, but we try to avoid any contact with anyone.”

“Of course,” Jupiter said with a sound of disappointment.

“I’m sorry, boys,” Commander Crane said, “I’d liked to have helped you. Can you find your way out of this cavern?”

“We’ve been trying to find our way,” Pete blurted out. “That’s what we were doing when we saw you.”

“Well, I think I can set you on the right track,” the commander said. “Remember, you must not talk about anything you have seen here involving our operation.”

“Yes, sir!” Pete agreed.

“Of course, Commander,” echoed Jupiter.

“All right, then, follow me.” The frogman led the boys to one of the tunnel openings, then through several caverns and side passages until they emerged into the large cavern where Pete had first seen the mysterious black and shiny shape.

“All right, boys,” Commander Crane said. “I expect you can make it from here. I must get back to my work.”

“Thank you, sir,” said Jupiter.

The frogman smiled at them. “And good luck with your work.”

He vanished back into the small opening, and Pete started for the tunnel that he remembered led out to Moaning Valley.

Jupiter made no move to follow. He was staring into empty space with that faraway look Pete knew only too well.

“Oh no!” Pete groaned. “Don’t tell me, Jupe.”

“I’m more certain that ever that we have to solve the mystery to-night, Pete,” Jupiter said. “The man disguised as El Diablo knew we would find our way out eventually. That means he didn’t care how much we knew as long as we couldn’t get in his way until some hours from now.”

“I don’t want to get in his way at all,” Pete observed, “but something tells me I’m going to.”

“This is our real opportunity, Pete,” Jupiter insisted. “Whoever is trying to scare people off thinks we’re still out of the way! We’ll never have a better chance to locate that digging and find out what makes the cave moan.”

“I guess you’re right,” Pete said dubiously. “Only maybe we’d better go and get Mr. Dalton and the other men first.”

“If we leave the cave we’ll be seen,” Jupiter pointed out. “Besides, there isn’t time. We’ve got to move fast now with the advantages we have.”

“Some advantage,” Pete said, “but I guess you’re right, Jupe. Where do we start? I mean, we’ve been here before, and we didn’t know which way to go.”

“But this time we have more information,” Jupiter said confidently. “This time we know that the digging has something to do with the moaning.”

“How do you figure that?” Pete asked, mystified.

“Because neither the sheriff nor the Daltons nor the newspaper mentioned a word about any digging. So whoever is digging is doing it in secret. By deduction, it has to be connected with the moaning sound because it is the only activity that goes on secretly in the cave when no one is here!”

“Well… ” Pete remained unconvinced.

“Two unexplained facts in the same place almost certainly must be connected,” Jupiter persisted.

Pete’s eyes widened. “Sure, okay. What do we do then?”

“First, you can use your keen sense of direction to find that side passage where we heard the digging.”

Pete nodded. Mentally he retraced their steps since they had been captured by El Diablo. At last he said, “Jupe, I figure we have to find a passage that will take us north-west.”

“That way,” said Jupiter, looking at his compass and pointing to the left.

“Right,” Pete confirmed. “Let’s go!”

The two boys lit their candles, forgetting their earlier caution in the excitement of being so near to a solution of the mystery. As they approached the opening in the north-west wall, a sound came out as if to greet them.

“Aaaaaahhhhhhhh — ooooooo — oooooo — ooo!”

“The moaning!” Pete whispered.

“It never stopped, Pete. We’ve just become accustomed to it.”

“It seems closer now.”

“Because it’s coming from that tunnel!” Jupiter held his candle in the tunnel entrance. A strong draught of air blew the flame, and with it the moan came again.

“Aaaaaahhhhhhhh — ooooooo — oooooo — ooo!”

The two boys plunged into the passage, which soon led into a small cavern.

“I know where we are now, Jupe,” Pete said in a low tone.

“Blow out your candle, then,” whispered Jupiter. “We’ll use our flashlights.”

The boys covered their flashlights with their hands so that only a faint glow could be seen, and Pete led the way into the same tunnel through which the fake El Diablo had taken them earlier. The moan grew louder as they walked.

“Aaaaaahhhhhhhh — ooooooo — oooooo — ooo!”

As they approached the cross passage, they heard the sound of digging.

“Golly,” Pete gulped. “We really did hear digging.”

“Of course we did. Come on!” whispered Jupiter.

They plunged ahead into the man-made tunnel, moving as silently as possible. It was long and straight. At the end of it they saw a glow of light. Jupiter motioned for Pete to slow down.

The glow of light came from a hole in the side wall of the mine shaft. Rocks and boulders lay in piles all around it. The sound of digging was coming from the hole.

Cautiously the boys crept forward and peered into the hole, blinking at the bright light.

At that moment the moaning came again — so loud that the sound was painful to their ears. It echoed all around them, then gradually died away.