“Twist it!” cried Ned.  “Twist it like a door knob.”

In a flash Tom did so.  For a moment no result was apparent, then, from somewhere far off, there sounded a low rumble, above the roar of the black stream.

“Something happened!” cried Mr. Damon.

“Back to the river!” shouted Tom, for they were some distance away from it now.  “If it’s rising we may have a chance to escape.”

They hurried to the edge of the stone channel, and Ned uttered a cry of delight.

“It’s going down!” he yelled, capering about.  “Now we can go on!”

And, surely enough, the river was falling rapidly.  It no longer roared, and it was flowing more slowly.

“The water is shut off,” remarked Tom.

“Yes, and see, there are steps which lead across the channel,” spoke Ned, pointing to them as the receding water revealed them.  “Everything is coming our way now.”

In a short time the water was all out of the channel, and they could see that it was about twenty feet deep.  Truly it would have been a formidable stream to attempt to swim over, but now it had completely vanished, merely a few little pools of water remaining in depressions on the bottom of the channel.  There were steps leading down to the bottom, and other steps ascending on the other side, showing that the river was used as a barrier to further progress along the tunnel.

“Forward!” cried Tom gaily, and they went on.

They went down into the river channel, taking care not to slip on the wet steps, and a few seconds later they had again ascended to the tunnel, pressing eagerly on.

Straight and true the tunnel ran through the darkness, the only illumination being their electric flash lamps.  On and on they went, hoping every minute to reach their goal.

“Dish suah am a mighty long tunnel,” remarked Eradicate.  “Dey ought t’ hab a trolley line in yeah.”

“Bless my punching bag!” cried Mr. Damon, “so they had!  Now if those ancients were building to-day—­”

He stopped suddenly, for Tom, who was in the lead, had uttered a cry.  It was a cry of joy, there was no mistaking that, and instinctively they all knew that he had found what he had sought.

All confirmed it a moment later, for, as they rushed forward, they discovered Tom standing at the place where the tunnel broadened out--broadened out into a great cave, a cave miles in extent, for all they could tell, as their lamps, powerful as they were, only illuminated for a comparatively short distance.

“We’re here!” cried Tom.  “In the city of gold at last!”

“The city of gold!” added Ned.  “The underground city of gold!”

“And gold there is!” fairly shouted Mr. Damon.  “See it’s all over!  Look at the golden streets—­even the sides of the buildings are plated with it—­and see, in that house there are even gold chairs!  Boys, there is untold wealth here!”