Mr. and Mrs. Dalton looked impressed. Jupiter’s correct use of so many big words always made adults think that he must be older than he actually was. Bob and Pete, however, were aware that Jupe had more on his mind than a walk along the beach. They tried to look as wide-awake as possible.

“Well… ” Mrs. Dalton began dubiously.

“Why not, boys?” Mr. Dalton decided. “It’s early, and I expect the first night on a ranch is too exciting to be wasted in sleep.” He turned to his wife. “Do them good, Martha. Better to look around the beach to-night, because I’ll be keeping them pretty busy starting tomorrow.”

“All right then,” Mrs. Dalton smiled. “Off with you, but no later than ten o’clock. We get up early here.”

The three boys did not wait for further talk. They carried their cookie plates and milk glasses to the kitchen and went out the back door.

As soon as they were out of the house, Jupiter started to give instructions. “Pete, you go to the barn and get that long coil of rope I saw hanging there. Bob, you go up to your room and get our chalk and our flashlights. I’ll get our bikes ready.”

“Are we going to the cave, Jupe?” Bob asked.

“Right. That is the only place to solve the mystery of Moaning Valley.”

“The cave?” Pete gulped. “Now? Couldn’t we see more in the day-time?”

“The moaning only happens at night,” Jupiter pointed out, “and when you’re inside a cave day-time doesn’t make any difference. Besides, the cave doesn’t moan every night. We know that it’s been moaning to-night, and if we don’t go now we might have to wait for days.”

By this time the other two were convinced. They moved swiftly into operation, and shortly afterwards the three boys met at the ranch gate.

Pete tied the long coil of rope to his luggage carrier, and they bicycled away along the narrow dirt road. The night was warm and the moon was up now, shining down on the silvery road that lay ahead.

Although the Crooked-Y Ranch stretched for miles along the shore of the Pacific Ocean, the sea itself was hidden by the coastal mountains. In the moonlight the rocky mountains were tall and silent, and the green live-oak trees looked like pale white ghosts. As they rode, the boys could hear the herds of cattle moving restlessly in the fields, the horses snorting and whinnying close to the road.

Then suddenly, without warning, the eerie moan floated out across the valley.

“Aaaaaaaaaaahhhhhhh — oooooooooooooooooo — oooooooo — oo!”

Even though they had expected it, Pete and Bob both jumped nervously.

“Good,” whispered Jupiter. “The moaning hasn’t stopped.”

They quietly parked their bikes and from the high ridge looked across the moonlit valley at the dark opening of El Diablo’s Cave.

“Gosh, Jupe,” Bob said. “I keep thinking I see things moving.”

“And I hear noises,” Pete added.

“Yes,” Jupiter said firmly. “But that is your imagination. In eerie surroundings like these, the simplest sound seems frightening. Now are we all ready? Bob, check the flashlights again.”

Bob tested the flashlights and Pete looped the rope over his shoulder. Each boy took his piece of chalk in his hand.

“Caves can be dangerous unless you take the proper precautions,” Jupiter explained. “The main dangers are falling into chasms and becoming lost. We have the rope in case any of us fall, and by marking our trail with chalk no one will become lost. We will stay together at all times.”

“Shall we mark our trail with question marks?”

“Right,” Jupiter said. “And we will also use arrows to indicate the direction we have taken.” The question marks in chalk were one of his most inspired inventions. The boys used them to leave a trail. The marks instantly made it clear that one of the investigators had been in a place. Since Jupiter’s chalk was white, Pete’s blue, and Bob’s green, it was even possible to tell exactly which one had made the marks.

“Well,” Pete said. “Are we ready?”

“I believe we are,” Jupiter said, satisfied at last.

The boys took a deep breath, and then began to walk down the slope of the ridge into the valley.

Once more the moan wailed out in the night “Aaaaaahhhhhhh — ooooooooooo — oooooo — oo!”

A swift current of cold air came to meet them as they neared the dark opening of the cave. Jupiter, in the lead, had already switched on his flashlight when suddenly he heard a rumbling sound.

“What’s that?” Bob cried.

The sound grew louder. Because of the strange echoing effect in the bowl-shaped valley, it seemed to come from all around them.

“Look up there!” Pete shouted, pointing.

A giant boulder was tumbling down the steep face of Devil Mountain in a shower of smaller stones.

“Jump!” Pete cried.

Bob hurled himself sideways out of the path of the hurtling boulder.

But Jupiter stood frozen, staring at the great rock as it fell straight towards him.

Pete threw himself at Jupiter, knocking the First Investigator away from the mouth of the cave. The boulder struck the ground with shattering force directly where Jupiter had been standing.

Bob scrambled to his feet. “Are you all right?” he asked anxiously.

Pete stood up. “I think so. Are you, Jupe?”

Jupiter got up more slowly and brushed at his clothes. His eyes had that faraway look that they always got when he was thinking.

“I was unable to move. A most interesting mental reaction,” he mused. “It’s similar to the way a small animal becomes paralysed when a snake looks at it. The animal literally can’t move, and is easily caught when it could have escaped.”

Bob and Pete both stared at their friend as he coolly analysed such a narrow escape from injury. Jupiter gazed up at the side of Devil Mountain in the moonlight.

“There seem to be many loose boulders up there,” Jupe observed, “and the mountain-side is very dry. I imagine it’s quite common for rocks to fall here. The naval gunnery probably loosened them in many places.”

The three boys approached the big rock. It was buried deep in the ground only a few yards from the entrance to El Diablo’s cave.

“Look, there are marks on it!” Bob was pointing at the boulder. “Gosh, Jupe, do you think someone pushed it down on us?”

“There are some marks,” Jupiter said after he had examined the rock more carefully. “Of course that is not so surprising.”

“It hit against a lot of other rocks on its way down,” Pete pointed out.

“We didn’t see anyone up there,” Bob said.

Jupiter nodded. “Still, someone up there might not want to be seen.”

“Gee, maybe we better go back,” Pete said.

“No, but we’ll be even more careful,” Jupiter said. “At least rocks can’t fall down a mountain at us when we’re inside the cave.”

With Jupiter in the lead, the boys entered the cave. They turned on their flashlights, and Bob marked the first question mark and arrow at the entrance.

Even with their flashlights on, they could see nothing but a long, dark passage that went straight into Devil Mountain. Its walls were smooth, and the ceiling was just high enough to permit Pete — the tallest investigator — to stand up. For a distance of about forty feet the passageway continued to be a straight tunnel with smooth stone walls. Then it suddenly opened into a large cavern.

The boys shone their flashlights all around the cavern. They were in a huge room with a towering ceiling. The far end of the cavern was so distant they could only just see it.

“It’s like a big city railroad station!” Bob exclaimed. “I never saw so big a cave.” His voice sounded hollow and faraway.

“Hello!” Pete called.

“Hello… hello… hello-ooooooo,” his voice echoed.

The boys laughed. The echoes seemed to bounce through the cavern. “Hello… hello-ooooo!” scouted Bob.