Had it not been for Tom Swift, the excited professor would have rushed pellmell over the jagged pile of rocks into the great cave which had been opened by the blast, the cave in which the scientist declared was the lost city for which he had been searching. But the young inventor grasped Mr. Bumper by the arm.  
"Better wait a bit," Tom suggested. "There may be powder gas in there. Some of it must have blown forward."
 
"I don't care!" excitedly cried the professor. "That is the hidden city! I'm sure of it! I have found it at last! I must go in and examine it!"
 
"There'll be plenty of time," said Tom. "It isn't going to run away. Wait until I make a test Tim, hand me one of those torches."
 
Some torches of a very inflammable wood were used to test for the presence of the deadly smoke-gas. Lighting one of these, Tom tossed it into the big excavation.
 
It fell to the stone floor—to the stone street to be more exact—and, flaring up brightly, further revealed the rows of houses as they stood, silent and uninhabited.
 
"It's all right," Tom announced. "There's no danger so long as the torch burns. You can go on, Professor."
 
And Professor Bumper rushed forward, scrambling over the pile of blasted rock, followed by Tom and the others. Some of the debris from the explosion had fallen into the cave, and was scattered for some distance along the main street of what had been Pelone. But beyond that the way was clear.
 
"Yes, it is Pelone," cried Professor Bumper. "See!"
 
He pointed to inscriptions in queer characters over the doorway of some of the houses, but he alone could read them.
 
"I have found Pelone!" he kept repeating over and over again.
 
And that is just what had happened. That last great blast Tom Swift had set off had broken down the rock wall that hid the lost city from view. There it was, buried deep down under the mountain, where it had been covered from sight ages ago by some mighty earthquake or landslide; perhaps both. And the earth and rocks had fallen over the main portion of the city of Pelone in such a way—in such an arch formation—that the greater part of it was preserved from the pressure of the mountain above it.
 
The outlying portions were crushed into dust by the awful pressure of the mountain—millions of tons of stone—but where the natural arch had formed the weight was kept off the buildings, most of which were as perfect as they had been before the cataclysm came.
 
The buildings were of stone block construction, mostly only one story in height, though some were two. They were simply made, somewhat after the fashion of the Aztecs. A look into some of them by the light of portable electric lamps showed that the houses were furnished with some degree of taste and luxury. There were traces of an ancient civilization.
 
But of the inhabitants, there was not a trace: either they had fled before the earthquake or the volcanic eruption had engulfed the city, or the countless centuries had turned their very bones to dust.
 
"Oh, what a find! What a find!" murmured Professor Bumper. "I shall be famous! And so will you, Tom Swift. For it was your blast that revealed the lost city of Pelone. Your name will be honored by every archeological society in the world, and all will be eager to make you an honorary member."
 
"That's all very nice," said Tom, "but what pleases me better is that this tunnel is a success."
 
"Success!" cried Mr. Damon. "I should call it a failure, Tom Swift. Why, you've run smack into an old city, and you'll have either to curve the tunnel to one side, or start a new one."
 
"Nothing of the sort!" laughed Tom. "Don't you see? The tunnel comes right up to the main street of Pelone. And the street is as straight as a die, and just the width and height of the tunnel. All we will have to do will be to keep on blasting away, where the main street comes to an end, and our tunnel will be finished. The street is over half a mile long, I should judge, and we'll save all that blasting. The tunnel will be finished in time!"
 
"So it will!" cried Job Titus. "We can use the main street of the hidden city as part of the tunnel."
 
"Use the street all you like," said Mr. Bumper, "but leave the houses to me. They are a perfect mine of ancient lore and information. At last I have found it! The ancient, hidden city of Pelone, spoken of on the Peruvian tablets, of gold."
 
The story of the discoveries the scientist made in Pelone is an enthralling one. But this is a story of Tom Swift and his big tunnel, and no place for telling of the archeological discoveries.
 
Suffice it to say that Professor Bumper, though he found no gold, for which the contractors hoped, made many curious finds in the ancient houses. He came upon traces of a strange civilization, though he could find no record of what had caused the burial of Pelone beneath the mountains. He wrote many books about his discovery, giving Tom Swift due credit for uncovering the place with the mighty blast. Other scientists came in flocks, and for a time Pelone was almost as busy a place as it had been originally.
 
Even when the tunnel was completed and trains ran through it, the scientists kept on with their work of classifying what they found. An underground station was built on the main street of the old city, and visitors often wandered through the ancient houses, wherein was the bone-dust of the dead and gone people.
 
But to go back to the story of Tom Swift. Tom's surmise was right. He and the contractors were able to use the main street of Pelone as part of their tunnel, and a good half mile of blasting through solid rock was saved. The flint came to an end at the extremity of Pelone, and the last part of the tunnel had only to be dug through sand-stone and soft dirt, an easy undertaking.
 
So the big bore was finished on time—ahead ............