“Gosh!” Pete whispered. “It hurts my ears.”

Jupiter caught Pete’s arm. “Look!”

Their eyes had adjusted to the bright light inside the cavern, and they could see a figure bending over with a shovel in his hand.

Pete gulped.

The figure suddenly straightened, put down his shovel, and picked up a pickaxe. For a moment he was clearly visible in the light of his electric lantern — a man with white hair and a flowing white beard.

Old Ben Jackson.

Through the opening in the side wall, Pete and Jupiter watched Old Ben working inside the secret cavern. Every few minutes, at irregular intervals, the moaning shattered their ears. The noise did not seem to bother the old man at all. He kept digging at the base of the cavern wall with his pickaxe.

“Look,” Jupiter whispered. “It looks like another rock fall.”

“It’s a big one,” Pete whispered back.

“You see how those breaks in the rocks are sharp and clean?” Jupiter pointed out. “That fall happened very recently.”

Old Ben continued his work at the fall, unaware of the eyes watching him. The old prospector swung his axe with vigour and surprising strength for a man of his age. Then he put it down again, and picked up the shovel.

“Jupe!” hissed Pete. “Look at his eyes.”

The eyes of the old prospector gleamed wildly in the light of his electric lantern, just as they had the previous night when the old man had warned them against The Old One.

“Gold fever,” Jupiter said softly, “or, in this case, diamond fever. I’ve read that prospectors often get like that when they think they have a strike. Nothing can get in their way or stop them.”

“Gosh,” Pete whispered.

Old Ben turned again to the wall and dug steadily at the fallen rock loosened by his pickaxe. He shovelled it into a kind of tilted sieve. Every few minutes, while the boys watched, he bent down and picked something out of the dirt. Each time, he examined the object, laughed wildly, and put it into a small leather bag near the electric lantern.

“Are they diamonds?” Pete whispered.

“I expect so,” Jupiter responded in the same low voice. Old Ben was so involved in his work that he probably wouldn’t have heard the boys if they had spoken normally, but they were taking no chances.

“Then he has found a diamond mine,” Pete said.

Jupiter was staring at the rock fall, his round face furrowed with thought.

“It looks that way, Pete, only — ”

“What else could it be? He’s struck a diamond mine, and he knows it’s on Crooked-Y property. If anyone found out about it he’d at least have to share the diamonds with the Daltons, wouldn’t he? Maybe legally they all belong to Mr. and Mrs. Dalton. So he only digs at night, and he scares everyone away from the cave!”

Jupe nodded slowly. “I guess you’re right, Pete. That explains everything except — ”

“Except why the cave moans,” Pete interrupted. “And what makes it stop when anyone comes inside.”

“I wasn’t thinking of that,” Jupiter said, “but I think I can explain why the moaning stops. You see, the sheriff and Mr. Dalton must have found this mine shaft all right. Only they didn’t find the place where Old Ben is working.”

As Pete opened his mouth to ask a question, a bell began to ring insistently in the hollow cave.

Old Ben dropped his shovel and moved with amazing speed to a small box near his lantern. He touched something on the box and the clanging bell stopped. Then he picked up his lantern and the small leather bag and headed straight for the hole in the wall where Pete and Jupiter were crouching.

“Quick, Pete!” Jupiter whispered urgently.

The two boys scrambled back to hiding places behind the loose mounds of rock in the shaft. They were none too fast. They had barely got out of sight when Old Ben came through the hole in the wall. The old man laid down his lantern and leather bag, and picked up a long steel bar the boys had not noticed in the floor of the shaft.

At that moment the moan began once more.

“Aaaaaahhhhhhhhh — oooo — ”

But this time the spooky sound stopped before it was finished. Old Ben had rolled a large boulder into the opening, using the bar as a lever. With the rock in place there was no trace of the opening. And the moaning stopped abruptly!

“Gosh, Jupe,” Pete said. “That’s what you meant! No one could tell there was a hole in that wall.”

The large boulder fitted tightly into the gap as if it had always been there.

“Right,” Jupiter whispered, “and the blocking of the hole stops the moaning right away. That bell must be the signal from whoever is watching up on the mountain. I think it means someone is coming into the cave.”

“Maybe Bob got scared and went for help,” Pete said. “I hope.”

Old Ben was pacing up and down in the mine shaft, muttering to himself. He didn’t even glance towards the rocks where the boys were hiding. Then, suddenly, the old man switched off his lantern. For a moment there was no sound in the dark mine shaft; then the boys heard the pacing and muttering begin again. They waited tensely in their hiding place.

In the darkness, Pete tried to sort out everything he had learned that evening. There were still questions he wanted to ask Jupiter, but Pete thought he understood most of the answers to the mystery of Moaning Valley.

Old Ben was digging secretly in the cave. Up on the mountain someone was on guard. The moaning sound was produced by the wind blowing through the narrow opening of the prospector’s secret cavern. When someone came to the cave, the guard signalled with the bell and Old Ben closed up the hole. The moaning sound stopped and there was no clue to what caused the moaning.

Pete felt quite pleased with himself for figuring it all out. He had answered all the questions himself… or had he? Who, for instance, was the fake El Diablo who had captured them? And how did he fit into the puzzle? Was that what Jupe had been referring to when he said something was still unexplained?

“Pete!” Jupiter’s voice whispered in his ear. “Someone is coming!”

Pete was so startled he almost lost his balance. He grabbed at the big rock in front of him, and a small stone rolled to the ground. Had Old Ben heard the noise? Pete held his breath.

A moment later he saw a bobbing light approaching.

“Waldo?” Old Ben’s voice said from somewhere nearby.

“Yup,” a voice replied from behind the bobbing light. “They’s two of ’em coming in the cave, Ben. We best skedaddle.”

Old Ben’s lantern came on, and Jupiter and Pete could see the tall, thin figure of Waldo Turner. The boys crouched as low behind the rocks as they could get. The two old men stood less than ten feet from them now.

“You sure they’re comin’ in?” Old Ben asked.

“I’m sure. Too dang many people foolin’ around this cave the last couple days,” Waldo replied.

“Jumping bobcats!” Old Ben exclaimed. “And I figure not more’n a few more days’ work ’fore we’re finished. Well, no sense gettin’ careless now. We better get on out.”

“We better,” Waldo agreed.

It was clear that Waldo Turner was the man on watch on top of Devil Mountain. After giving the alarm, he had come down by some secret passage from the top.

The boys watched the two old prospectors move the boulder away from the hole, pass quickly through, and lever the rock back into the hole from the inside. Then there was silence in the pitch-black mine shaft.

“Where did they go, Jupe?” Pete whispered.

“There must be an exit from that cavern to the outside of the mountain. There would have to be. The wind wouldn’t be able to make that moaning sound if it couldn’t blow through from the other side. It’s probably one of those old mine shafts that are supposed to be sealed up. I’ll bet Old Ben and Waldo knew just where those shafts were and unsealed one.”