Suddenly Tom, who, with Professor Bumper, was in the lead, uttered a cry, as he held his torch above his head and flashed it about in a circle.

“We’re blocked!” he exclaimed.  “We’re up against a stone wall!”

It was but too true.  Confronting them, and extending from side to side across the passage and from roof to floor, was a great rough stone.  Immense and solid it seemed when they pushed on it in vain.

“Nothing short of dynamite will move that,” said Ned in despair.  “This is a blind lead.  We’ll have to go back.”

“But there must be something on the other side of that stone,” cried Tom.  “See, it is pierced with holes, and through them comes a current of air.  If we could only move the stone!”

“I believe it is an ancient door,” remarked Professor Bumper.

Eagerly and frantically they tried to move it by their combined weight.  The stone did not give the fraction of the breadth of a hair.

“We’ll have to go back and get some of your big tunnel blasting powder, Tom,” suggested Ned.

As he spoke old Goosal glided forward.  He had remained behind them in the passage while they were trying to move the rock.  Now he said something in Spanish.

“What does he mean?” asked Ned.

“He asks that he be allowed to try,” translated Professor Bumper.  “Sometimes, he says, there is a secret way of opening stone doors in these underground caves.  Let him try.”

Goosal seemed to be running his fingers lightly over the outer edge of the door.  He was muttering to himself in his Indian tongue.

Suddenly he uttered an exclamation, and, as he did so, there was a noise from the door itself.  It was a grinding, scraping sound, a rumble as though rocks were being rolled one against the other.

Then the astonished eyes of the adventurers saw the great stone door revolve on its axis and swing to one side, leaving a passage open through which they could pass.  Goosal had discovered the hidden mechanism.

What lay before them?

CHAPTER XXV

THE IDOL OF GOLD

“Forward! cried Tom Swift.

“Where?” asked Mr Damon, hanging back for an instant.  “Bless my compass, Tom! do you know where you’re going?”

“I haven’t the least idea, but it must lead to something, or the ancients who made this revolving stone door wouldn’t have taken such care to block the passage.”

“Ask Goosal if he knows anything about it,” suggested Mr. Damon to the professor.

“He says he never was here before,” translated the savant, “but years ago, when he went into the hidden city by the cave we left yesterday, he saw doors like this which opened this way.”

“Then we’re on the right track!” cried Tom.  “If this is the same kind of door, it must lead to the same place.  Ho for Kurzon and the idol of gold!”