"Sleep nothing! I've just thought of it. That tunnel entrance is
INSIDE the temple. That's the most natural place in the world for
it. I'll bet it's right in the middle of the big inner chamber,
where the priests could control it. Why didn't we look there
before?"

"That's right; why didn't we?" agreed Tom. "I believe you're right,
Ned! We'll look the first thing in the morning."

They did not wait for breakfast before trying the experiment, and
Mr. Damon and Eradicate went with Tom and Ned. It was no easy work
to make their way over the ruins to the inner auditorium. Wreckage
and ruin was all around, and they had to avoid the yawning holes on
every side. But when they got to the main, or sacrificial chamber,
as Ned insisted on calling it, they found the floor there solid. In
the centre was a great altar, but to their chagrin there was not a
sign of a tunnel opening.

"Fooled again!" said Tom bitterly.

"Maybe some of those holes outside is the entrance," suggested Mr.
Damon.

"I don't believe so," objected Tom. "They seemed to go only to the
cellar, if a temple has such a thing."

Bitterly disappointed, Tom strolled over and stood in front of the
big stone altar. It seemed that he must give up the search. Idly he
looked at the sacrificial stone. Projecting from it was a sort of a
bundle.

Tom took hold of it, and to his surprise he found that it could be
moved. Hardly knowing what he was doing, he pulled it toward him.

The next instant he uttered a cry of horror, for the immense stone
altar, with a dull rumbling, rolled back as though on wheels, and
there, over where it had stood was a hole of yawning blackness, with
a flight of stone steps leading down into it. And Tom stood so near
the edge that he almost toppled in.

"Look! Look!" he cried when he could get his gasping breath, and
step back out of danger.

"The tunnel entrance!" cried Ned. "That's what it is! You've found
it, Tom! The entrance to the city of gold at last!"

CHAPTER XIX

THE UNDERGROUND RIVER

They gathered around the opening so unexpectedly disclosed to them,
and stared down into the black depths. Beyond the first few steps of
the flight that led to they knew not where, nothing could be seen.
In his impatience Tom was about to go down.

"Bless my match box!" cried Mr. Damon. "What are you going to do,
Tom, my boy?"

"Go down there, of course! What else? I want to get to the
underground city."

"Don't!" quickly advise