There were the footprints of two men just as they descended the stairs. But there were no return marks.

“They’re down there yet,” declared Frank, positively.

Randall looked at him keenly.

“Is this another entrance to the volcano?” he asked.

The young inventor shook his head.

“I think not,” he said, “though what should detain them down there I do not understand.”

“Well, suppose we go down?”

“Certainly.”

The three men went quickly down the staircase. They were soon in the passage which had been followed by Barney and Pomp.

But they did not follow it far. Their progress was checked. A great wall of earth and stone confronted them.

At once the truth flashed upon Frank.

“A cave-in!” he declared. “They are imprisoned!”

Randall was deadly pale as he turned to Frank.

“My soul! You don’t think they are under that debris?”

“Let us pray not!”

“What shall we do?”

“There is but one thing!”

Frank threw off his coat. Then he turned to the stairs.

“Where are you going?” asked Randall.

“After shovels and picks. We must do some hard digging. I shall not leave here until I have brought them out dead or alive!”

“Amen!” cried Randall. “I am with you, Frank!”

In less time than it takes to tell it the tools were brought and work begun. And it was at this juncture that Barney and Pomp heard their deliverers.

At once they grasped the truth, and Barney joyously cried:

“Whurroo! we’re goin’ to git out of here, naygur, shure. It’s Misther Frank afther us!”

Then the two imprisoned fellows went to work like beavers. In a short while they were able to shout and be heard on the other side.

The rest was easy.

Before long they crawled out of their captivity, and none too soon, either, for the air was getting extremely foul and dangerous.

But soon they were above ground and safe. It was a joyful moment for all.

Further exploration of the ruined town was made, but nothing of great interest was discovered, and finally Frank concluded to go on. So all went on board the Scorcher, and it rolled away across the Polar country.

Everywhere was that same desolate, abandoned appearance. What had become of the Polar people, it was not easy to guess.

Cities and towns to the number of a dozen were encountered in the next week. Then, the explorers came to a high mountain range, which Frank declared marked exactly the locality of the South Pole.

It must have been ten or twelve thousand feet in height, and was all of solid granite.

Sheer from the green plains the mountain walls rose to a dizzy height. It was a stupendous sight.

Nowhere did they seem possible of ascent. But as he studied them an idea occurred to Frank.

What was on the other side of them?

Was there a fertile region like this, or was it a desert waste? Who could say that the mysterious disappearance of the Polar people was here capable of explanation?

Perhaps they bad abandoned the region this side of the range for a land of milk and honey on the other. Frank had a powerful desire to see what was on the other side of that impenetrable and insurmountable wall.

But he saw no easy way of scaling it. It was shut in on both sides by an equal wall, extending for over a hundred miles in both directions.

Mystery—mystery! This was in the very air of the abandoned country. He was unable to solve it.

The Scorcher traveled along the wall for several days. But there was no break which would allow the machine to cross it.

“Well, I’m beat,” muttered the young inventor at last. “This beats all the puzzles I ever attempted.”

“It’s a mighty curious part of the world, mates,” declared Wendel.

“I agree with you,” said Frank, “but there must be some explanation of the mystery.”

And he continued to grope for it. But the days passed and he was no nearer success than ever.

Meanwhile the Antarctic night was wearing on.

While the sky remained clear of clouds the semi-gloom of the landscape was not bad. But when clouds obscured the heavens, then at times the darkness was most intense.

At such times it was often necessary to abandon the quest and wait for the darkness to pass.

The searchlight, of course, would dispel the gloom, but as it would be slow work pursuing research at such a time, Frank suspended all operations.

And thus time wore on.

But thrilling events were in store.

One day the Scorcher rested at the base of the high mountain wall. Frank and Randall had left her for a walk over the green turf.

Randall was an expert g............