Bob had disappeared.
“Bob!” Jupiter stepped towards the other side of the cave and then stopped in his tracks, dumbfounded. “What happened?” Pete asked. He was on his feet, staring.
Jupiter could only point to the cave wall they were facing. “He was here a second ago. Didn’t you see him? Then the wall seemed to swallow him up.”
“What?” Pete leaped headlong for the wall. It brought him up short. “I don’t get it,” he muttered, shining his torch on the floor. “There’s no pit this time.”
As he stooped to examine the floor of the cave, there was a rumbling noise. Pete’s eyes widened and he gripped his heavy torch tighter. He looked up at Jupiter and was surprised to see the stocky boy smile.
“It’s okay,” Jupe said. “He’s coming back now.”
Pete whirled in time to see the movement in the wall. A small section was turning. The next moment there was a gaping hole where there had been a wall, and Bob was on his knees crawling through.
“How about that!” Bob exclaimed. “Whiskers! A secret moving rock panel. I just happened to lean on it and — wham! It opened!”
“What’s on the other side?” Jupe asked excitedly.
Bob’s jaw dropped. “I didn’t get a chance to notice, Jupe. It happened too fast. Let’s see if I can open it again.”
He backed against the low cave wall, squatting. Nothing happened at first, and he shifted his shoulders.
Suddenly, they heard a sharp clicking sound. Then the rock rumbled and Bob tilted back.
“Here I go again,” he cried. “Better use it while it’s open!”
He tumbled through the opening, and then Pete and Jupe slid through.
“Wow!” Pete exclaimed. “This is more like it!” This cave was very wide and high. It extended back as far as their torches could reach, running inland parallel to the first cave they had entered. The Three Investigators jumped to their feet to examine it.
As they did, they heard a slight rumbling noise behind them. They whirled, too late.
The secret rock opening had closed behind them! “Uh-oh!” Pete muttered. “That does it!”
“That’s what happened to Bob before. I’m sure we’ll be able to work out how it works later,” Jupe said. “Probably a simple leverage system. Let’s forget it now while we investigate this cave.”
Bob looked up at the high vaulted ceiling.
“Wow!” he gasped. “Look at the size of it, Jupe This might be the tunnel I read about!”
Jupiter nodded. “Possibly, Bob. But if you notice, the walls and ceiling are rough, unfinished rock, as any cave is. The tunnel you told us about was completed. It would be in a more finished condition, with concrete walls possibly and a paved flooring. Perhaps tracks, too. Or the bedding for them.”
He shook his head, and flashed his light about. “No, this looks like any large natural cave. There’s no entrance to the beach and water behind us, either. The walls appear solid all the way round. But let’s follow its course under the cliff. It may still lead us to the old underground railway tunnel you mentioned.”
“What I like about it,” Pete said, “is that there’s no way out to the beach. That means, there’s no way in for things like dragons!”
“Which means we’re in luck,” Jupiter said smiling.
“There’s one thing certain about this cave anyway. It happens to be large enough to contain a dragon or any other creature that size.”
“Thanks for reminding me,” Pete muttered. “I nearly felt good there for about half a second.”
The cave floor was fairly smooth and level and they advanced along it steadily. Then they stopped short.
The passage ended suddenly in a high, vertical grey wall.
“End of the road,” Pete said. “Looks like all we’ve discovered is the world’s biggest unused parking lot!”
Jupiter was pinching his lower lip and looking puzzled.
“What’s wrong, Jupe?” Bob asked.
“That wall ahead of us,” Jupe said. “There’s something about it that doesn’t look quite right.”
Bob and Pete flashed their lights on it, and shook their heads.
“It looks like a wall to me,” Bob said. “Naturally I’m as disappointed as you are. I wanted — ”
Jupiter had his eyes half closed and wasn’t listening. He peered closely at the wall and tapped it. Then he hit other parts of it, his ear close to the grey surface.
“It sounds funny, Jupe,” Bob said.
Jupiter nodded, frowning, then walked to the side of the cave and tapped the wall there.
“There’s a difference,” he said finally. “I can’t explain it exactly but — ”
“Oh, come on, Jupe,” Pete interrupted impatiently.
“If you can’t prove it’s not a wall, then it is. Let’s get out of here. I’m getting cold.”
Jupiter’s face lit up. “That’s it!” he exclaimed.
“Cold! The wall isn’t cold. But the sides of the cave are. Feel the difference for yourselves.”
Bob and Pete hastily compared the feeling of the wall and the sides of the cave.
“You’re right,” Pete admitted. “It’s not as cold as the sides of the cave. But what does that prove? Don’t forget it’s under some of those houses along the ridge. Maybe heat comes down from them and warms the wall a little.”
“Heat rises, Pete,” Jupe said.
“There may be another pocket or passage inside,” Bob suggested, “That could warm it, too, Jupe.”
Jupiter shook his head, his mouth forming the stubborn line they were accustomed to seeing when he disagreed with them.
He took out his pocket knife and started to scrape at the rough grey slab.
Pete laughed. “You’ll only ruin your blade trying to cut through that rock, Jupe. What you need is a small stick of dynamite.”