Jupe looked towards the next staircase. It was too far away, requiring a long run over heavy sand that would slow them down. They’d be an easy target on the open beach.

Quickly he made his calculation. “We’ve got only one chance. Quick! Back into the cave!”

The boys veered off, changed direction and sprinted for the cave opening. They ran desperately, expecting at any moment to hear the sharp retort of the spear-gun.

Or perhaps to feel the long and deadly steel shaft it propelled.

The sand flew under their feet.

“Almost there!” Jupiter gasped. “Dive!”

The three leaped headlong for the mouth of the cave, almost as one. Then they scurried on hands and knees behind the big boulders for protection.

“Made it!” Pete grunted. “Now what?”

“We hide,” Jupiter said, struggling to catch his breath. “It will give us time — to work out a plan.”

“Maybe now’s the time to look for those other passages,” Bob said.

Jupiter nodded, his cheeks pink with excitement. “Agreed, Bob. But we’ll let him make the first move. If he comes this way, then I’ll admit our situation demands emergency procedure — such as going deeper into this cave.”

Pete was peering over Jupiter’s shoulder. His voice was tight. “We’ll have to, Jupe — here he comes!”

“Whiskers!” Bob exclaimed. “What’ll we do? I don’t feel like dropping into that pit again.”

Jupiter had backed into the cavern wall.

“Look!” he suddenly called to them.

In the wall, they saw some vertical planking, running up to the ceiling of the cave.

“Jeepers,” Pete said. “How did we miss that before?” “The sand and dust provided a natural covering,”

Jupiter said. He tapped the panel with his hand. It brought a hollow sound.

“It must be a secret passage,” he said. He pushed at the boards. “They appear loose enough to be moved. Take another quick look, Pete. See if that skin diver is still coming for the cave.”

Pete peered out and quickly ducked back.

“We’ve got double trouble,” he said, his voice shaking. “Now there are two of them!”

Jupiter frowned. “Two? In that case, we’d better work fast. Help me with these planks.”

They tugged and pushed at the planks.

“It’s no use,” Bob said. “They’re wedged in too tightly.”

Jupiter shook his head. “There has to be a way.” His eyes gleamed suddenly. “Of course! Stupid of me.”

He kicked at the loose sand under the plank. “All we have to do is dig down a little. Get some of that sand out. That will loosen them up.”

Quickly, they dropped to their knees and scooped away at the sand. The wide plank above it suddenly moved.

“That’s it,” Jupiter said. “Now, if we can turn it enough for us to slip through — ”

Once they had disturbed its base, the plank was easy to manage. Bob and Pete went through. Then it was Jupiter’s turn.

He struggled to get through the narrow opening. “Can’t — do it,” he gasped. “Still — too fat!”

Bob and Pete hastily scooped out more sand below, from the other side. The plank swung open, and Jupiter heaved himself through.

“Let’s leave a little crack to see through,” he whispered.

They swung the heavy board back into place, making sure it didn’t fit tightly against its neighbour.

They were still on their knees in the dark pocket of the cavern, when they heard voices.

The first skin diver flicked on a torch.

“I could have sworn those kids ran in here, Harry. Too bad you let that wave knock you down and I had to take my eyes off them.”

“We’ll find them soon enough, if they’re in here,” the other answered. “If they’re not, we can get to work.”

The Three Investigators held their breath as the first man played his torch round the cave. Jupiter, on hands and knees, pressed his face to the opening between the long boards.

Bob and Pete stooped over him, each with an eye at the crack.

The men in the black wetsuits walked ahead. The light faded, as did the sound of their shuffling flippered feet.

The rasping voice of the second man echoed from the pit area. “You must have been imagining it, Jack. Nobody here.”

“Guess they got away up the other steps, then.”

There was a faint plopping sound and then silence. Jupiter could hear and see nothing more, and drew his face back from the board. Dust and sand were making his nose itch. He wondered if his partners had the same symptoms. A sneeze would be a disaster for them now.

“No sneezing,” he whispered. “Cover your noses.”

They obeyed his warning, and waited nervously. The cave remained dark and silent. Finally Jupiter stood up.

“They’ve gone,” he whispered. “Let’s get out while we can.”

They scooped away the loose sand again, and carefully eased back the plank.

“You first this time, Jupe,” Pete whispered. “If you get through, Bob and I can make it easily.”

Jupiter took the suggestion with a smile.

They slipped back into the mouth of the cave and listened. The cave remained silent. They put the board quietly back into place, and brushed in sand until it stood firm again.

Jupiter stood up, his heart beating wildly. He looked at his watch.

“We’ve had over three hours,” he whispered. “Hans will be waiting.”

“Well, what do you make of it?” Jupiter asked.

It was an hour after their return with Hans in the truck. Bob was at home getting a much needed shower and change of clothing. Pete and Jupiter were alone in Headquarters.

Pete shrugged. “I can’t figure it out. I don’t know who those skin divers were, except that their names were Harry and Jack. I don’t know why Harry, or Jack, pointed that speargun at us. I don’t know why they followed us into the cave. I don’t know where they disappeared to or how. I don’t even know how we got out of there alive.”

Jupiter pinched his lip, and nodded. “Add to that the curious incident of the collapsing staircase, and it’s obvious we’re confronted with many questions before we try to solve the mystery of Mr. Allen’s missing dog.”

“I’ve got an idea that will help,” Pete said.

“Oh?” Jupiter wheeled round in his chair, his eyes sparkling with interest. “What?”

Pete gestured to the telephone on their desk. “Just pick up that telephone and call Mr. Allen. Tell him we’ve decided not to find his missing dog for him. Tell him we nearly disappeared ourselves. Tell him we’re willing to forget the whole thing.”

Jupiter ignored the suggestion.

“Our first problem,” he stated, “is to determine who those skin divers were, and what they were doing in the cave.

Pete shook his head. “Why bother with those two tough characters? We were in there, too, and I don’t even know what we were doing in there.”

“Looking for clues of Mr. Allen’s dragon,” Jupiter replied. “And tracks or traces of his Irish setter, Red Rover.”

“Well, we didn’t find much,” Pete declared. “Except that pit. Bob found that for us.”

“We found that passage behind the boards,” Jupiter said. “It may be a secret tunnel in the cave. Or it could be one of the secret hiding places used by the rum-runners or smugglers in the old days.”

“I don’t see what that’s got to do with us,” Pete answered stubbornly. “It wasn’t hiding Mr. Allen’s dog.”

Jupiter frowned. “As investigators, we have to return and examine that cave more thoroughly. Don’t you realize that?”

Pete nodded reluctantly. “Sure. All I’m wondering about is how those skin divers didn’t fall into the pit Bob found? Doesn’t that prove they know their way around in there?”

“Maybe, but they did have a torch,” Jupiter said. “As to how or why they disappeared, perhaps when we return with torches, we’ll discover — ”

The telephone rang, for the second time that day. They stared at it.