"But where did they go after they got down in the pa.s.sage?" asked Belle.

"They couldn't stay there all the while, and there's no sign of any one there now, unless they're invisible. They couldn't get past the blocking cement wall."

"There's something beyond that wall, and we're going to find it!"

declared Cora.

"Perhaps when the boys come back they won't have permission to tear it down," suggested Bess.

"Then we'll begin our investigations from the other end," Cora said.

"What other end?" Hazel questioned.

"The cave! I think this pa.s.sage connects with the cave. That would explain a hiding place for whoever has been playing these tricks on us, and making that strange noise."

"You mean the cave you accidentally discovered yesterday?" asked Belle.

"Yes," Cora answered. "I'm sure that has something to do with the mystery. So if Mr. Haight won't let us open the wall, we may be able to see what is on the other side by going to the cave, and finding the pa.s.sage that connects with the one which comes out into our bungalow."

"In that event the stone wall must be movable," suggested Hazel.

Cora jumped up so suddenly that she disturbed Bess who was leaning against her.

"That's it! That's it!" Cora cried. "I wonder we didn't think of it before. That surely is it!"

"What is?" eagerly demanded Belle. "You are talking in riddles."

"This whole affair is a riddle, girls!" exclaimed Cora. "But what Hazel said gave me an idea. That cement wall seems solid, but it can't be. If it were no one could pa.s.s. So it must be made to look solid to deceive those not in the secret. Probably it is a balanced stone like the ones you read of in stories of the cave dwellers. Some of them closed the entrances to their caves by heavy rocks, set on pivots, turning when you pressed on a certain mechanism. There are counterweights, just as in a window, which makes the heaviest rock move easily. I'm sure that's what is in the pa.s.sage-a balanced rock doorway. And there won't be any need of tearing the wall down at all!"

"It sounds like a detective story," commented Bess.

"It may turn out to be one before we're through," Cora said.

"Oh, dear! Why don't those boys hurry back?" cried Belle for perhaps the tenth time. "Let's go out and look down the road to see if they are coming."

The girls went out, too anxious and too eager to sit still, but they had no sight of Jack and his chums.

"I'm not going to wait any longer!" exclaimed Cora at length. "If I'm right, there will be no need of tearing down the wall. That is, if I can find the mechanism that turns the rocky door. And if I'm wrong, there won't be any harm in doing it."

"Doing what?" asked Bess.

"Going down into the pa.s.sage to see what we can discover. Will you come?"

"I will!" exclaimed Hazel.

"Then we will, too, Belle," said Bess, quickly. "They shan't call us cowards, even if we are twins."

"Come on," cried Cora gaily. "We must do something or fly to pieces with nerves. Anything is better than sitting still, waiting."

Back to the bungalow the girls hastened. The hole in the floor was still open, the sliding door having been braced back so it could not be closed by any accident.

"We don't want to go down there and not be able to get up again," Cora remarked.

"Shall we tell Mrs. Floyd we're going to see what we can find?"

suggested Belle.

"No," decided Cora, after a moment's thought. "She might not want us to until we have Mr. Haight's permission. We'll just do this on our own responsibility. We won't damage the wall any."

Down the cement steps went the four girls, into the dark pa.s.sage. The boys had left behind their flashlights and these were carried by Cora and Hazel. The small, but powerful lamps gave a good light.

"Ugh! It's rather creepy in here," complained Belle, looking back over her shoulder.

"Don't be silly!" said Cora, sharply. "Just think of it! We may solve the mystery all by ourselves."

"Let us hope so," murmured Bess.

They came to the cement wall that barred further progress along the pa.s.sage.

"First to see if it's solid rock," Cora suggested.

"How?" asked Belle.

"By tapping. I brought along a hammer." With this implement Cora gave several blows against the obstruction. An unmistakable hollow sound resulted.

"I believe it's only a wooden door, covered over with cement stucco,"

Cora said. "Now to find the secret catch."

With their flash lamps the girls went over every inch of the surface of the door. At first it all looked alike, dull, gray, smooth cement. Then Cora's light lingered a moment on a certain place.

"Girls, watch!" she said suddenly.

With her thumb she pressed on the spot where it seemed the cement was worn smoother than elsewhere. And then, to the surprise of even Cora herself, the cement door swung slowly back, revealing a dark pa.s.sage beyond.

"We've discovered it!" cried Cora. "No need to tear the door down. Just as I suspected, it's on hinges. Come on, girls!"

Hardly realizing what they were doing, the others followed, their torches illuminating the plank sides of the underground tunnel.

"Are you sure this is all right, Cora?" asked Belle, as she stepped beyond the open door.

"I'm not sure anything is right about all this mystery," was the answer, "but this is a chance we mustn't miss. Come on."

"But where are we going?" Belle queried.

"I can't tell, but I think to the cave," Cora answered. "Don't be afraid. There's no danger."

Hardly had she spoken than a noise sounded behind them.

"Oh! what's that?" cried Belle, rushing forward.

"It sounded like a door closing," said Hazel.