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 advised the odd man.  “You don’t know what’s there.  It may be a trap, where the old Aztecs used to throw their victims.  There may be worse things than bats there.  You’ll need torches—­ lights—­and you’d better wait until the air clears.  It may have been centuries since that place was opened.”

“I believe that’s right,” agreed Ned.  “Whew; Smell it!  It’s as musty as time!”

An unpleasant odor came up the tunnel entrance, and it was stifling to stand too close.  Tom lighted a match and threw it down.  Almost instantly the flame was snuffed out.

“We couldn’t live down there a minute,” said the young inventor.  “We’ve got to wait for it to clear.  We’ll go back to the balloon and get some electric flash lamps.  I brought along a lot of ’em, with extra strong batteries.  I thought we’d need some if we did find the city of gold, and it looks as if we were almost there now.”

Tom’s plan was voted good so they hurried out of the temple, their feet echoing and re-echoing over the stone floor.  The place, ruined and desolate as it was, had no terrors for them now.  In fact they were glad of the very loneliness, and Tom and Ned actually looked about apprehensively as they emerged, fearing they might see a sign of the Mexicans or the Fogers.

“Guess they can’t pick up our trail,” said Tom, when, he saw of what Ned was thinking.

“No, we’ve got the place to ourselves.  I wonder how long it will take for the air to get fresh?”

“Not so very long, I guess.  There was a good draught.  There must be some opening in the underground city by which the air is sucked in.  They’d never have only one opening to it.  But we don’t need to look for the other.  Come on, we’ll get out the torches.”