Certainly the appearance of the volcanic mountain was unusual in the extreme. What did it mean?

Had internal fires burned it out and made of it a hollow cone? It certainly looked very much so.

But now another startling thing was seen. Into the vast cavity a large body of men were seen to be rushing.

“It is the home of the barbarians!” cried Professor Gaston, in amazement. “More and more wonderful!”

The aerial voyagers gazed upon the spectacle in sheerest wonder.

Into the mighty aperture rushed the Antarctic natives. In a few moments not one was in sight.

The airship now rapidly settled down at the foot of the volcano.

There was one resolute purpose in the minds of all.

They were determined to invade the curious dwelling place of the natives. It was a moral certainty that the white prisoners, Lucille and Mark Vane and Alvan Bates were therein confined.

This being the case, there was sufficient excuse for the invasion, for it was necessary to rescue them.

The airship descended until on a level with the cavernous opening. It could easily have sailed into the place, but Frank was afraid that collision with the roof might damage the wings or rotascope.

So he did not venture to enter.

But getting down on a level he turned the rays of the searchlight into the place. This revealed a curious sight.

A mighty open space, or perhaps it might be called cavern, occupied several acres in extent, and all roofed by the shell of the volcano.

But in the centre of this vast underground area was what looked like a lake of molten gold as it lay under the gleam of the searchlight.

However, Frank saw that it was nothing of the kind, but a vast basin of boiling lava.

A stream of the boiling liquid ran down into the basin from an orifice in the mountain wall.

The walls of the immense cavern were of hardened lava, apparently. It was certainly a queer freak of nature.

But this was not all.

The Antarctic natives had entered the place, but none of them were in sight.

Frank was in a position whence he could easily view the whole interior of the place.

But an explanation of their disappearance was easily obtained.

Just beyond the lava basin there was a dark, cavernous opening which appeared to trend downward.

Frank understood it all at once.

“I have it!” he cried. “This is only one of many caverns in this volcanic range. The whole region here doubtless is honeycombed by the action of currents of lava. Doubtless their retreat is deep down in the bowels of the earth.”

Captain Hardy heard this with dismay.

“Then we can never hope to rout them out!” he said. “That will not be possible.”

“On the contrary, I believe it is possible,” said Frank.

“You do?”

“Yes.”

“How will you do it?

“Easiest thing in the world. Simply track them right into their den.”

Captain Hardy shrugged his shoulders.

“You cannot go there with your airship,” he said.

“Very true!”

“How then do you propose to go?”

“On foot.”

“Mercy! a handful of men like us will stand no show with such a myriad of foes, however insufficiently armed.”

“How many of the natives do you reckon there are?” asked Frank.

“At least several thousand.”

The young inventor was silent. He realized that there was logic in Captain Hardy’s words.

But he was not to be defeated.

“Barney,” he said, “go down and fetch up those long, black boxes in the forward cabin.”

“All roight, sor!”

The Celt disappeared at once.

When he returned he had two of the boxes on his shoulder. They were marked in plain black letters:

“Armor!” exclaimed Captain Hardy. “Is that what you have there, Mr. Reade?”

“That is it,” replied Frank.

“Mercy on us! I supposed the days of armor and knighthood had gone by.”

“Neither have as yet,” replied Frank, quietly. “I have four suits of this armor, and it is my own manufacture. Did you ever see anything better?”

As Frank said this he took from one of the boxes a shirt of mail.

The finest of steel meshes, intricately woven, and all as pliable as cloth. Such was the wonderful armor.

There was a suit from head to foot, including a helmet, with visor and skull cap. Truly it was wonderful workmanship.

“It is bullet proof,” declared Frank. “Nothing ordinary can penetrate it.”

“Wonderful!” cried Jack Wallis. “Why, with this armor one man could hold an army at bay.”

“That he could,” agreed Frank. “They might fire volleys at him. They could not kill him.”

The suits of mail were carefully examined and admired.

Then Frank said:

“You get into one, Wallis; and you, Captain Hardy, into the other. Pomp will remain with the machine. Barney, don this suit of mail and at once.”

“All right, sor!” replied the Celt, who proceeded to obey.

“Then you propose to wear these suits of mail in attacking the natives?” asked Hardy.

“Certainly,” replied Frank. “Thus equipped we can clean out the country. Ah, there is great work ahead for us!”

All were, of course, enthusiastic over the prospect.

It is needless to say that they were soon ready. Over the rail they went and stood upon the volcanic ground.

Pomp elevated the airship a few hundred feet for safety’s sake, after they had gone. Then the four rescuers entered the hollow mountain.

As they did so they noted a peculiar vibration and at times a distant jarring, jolting sound as if machinery were at work beneath them.

And doubtless it was, but not machinery made by human hands.

The internal fires raging there, no doubt, caused the tremulous motion. Indeed, the atmosphere was charged with waves of heat, which was evidence enough in itself of that.

Entering the hollow mountain, the four mail-clad men skirted the lake of molten lava.

The heat from this was something not exactly pleasant to bear. They did not venture too near the edge.

Upon every hand was visible evidences of the great struggle of the volcanic elements in ages past.

It was a wonderful sight, and Professor Gaston made the best of it. He declared:

“I am the most fortunate man in America to-day to be enabled to be here. This is a wonderful experience!”

As the professor had not a suit of armor on it was decided that he should remain in the outer cavern where he would be very much safer.

He was anxious to search for specimens, and at the same time was not desirous of an encounter with the natives.

Leaving Professor Gaston in the outer cavern, Frank Reade, Jr., and his three companions boldly entered the subterranean passage which led presumably to the stronghold of the Antarctic natives.

To their surprise the passage was hardly a hundred feet in length.

Then they emerged upon a scene the like of which none of them had ever before beheld. It was wonderful.

They emerged upon a long gallery, from which they looked down into an internal crater full two hundred feet deep.

A mighty basin it was, covering acres with small islands of rock in a vast lake of fire and lava.

Great sheets of burning gas at times leaped a hundred feet into the air. Yet certain draughts of air made the gallery secure against the frightful heat.

For some while our explorers gazed upon the scene with wonder.

“Upon my word!” exclaimed Captain Hardy. “Inferno could not be worse than that!”

“You are right,” agreed Frank. “Certainly it is akin to it.”

“Begorra, I’d niver want to fall down there!” cried Barney, with a shiver. “Shure, it’s moighty quick yez would come to nothing.”

Nobody was disposed to contradict this logical statement. But Jack Wallis was impatient.

“If we are to save the captives I think we had better move,” he said.

Everybody agreed to this, and they now pressed forward along the gallery.

For perhaps a hundred yards this followed a winding way, and suddenly a startling view burst upon the rescuers.

Daylight was visible just ahead, and now they emerged into a narrow and deep valley right among the peaks.

What was the most striking was that this valley was as green as an emerald, which, indeed, it seemed like in a rough setting of mighty jagged heights.

Vegetation flourished in this peculiar valley. There were larches, cedars and spruces, and a peculiar sort of grass interspersed with moss turfed the valley.

This was the home of the Antarctic people. Truly it was a remarkable spectacle.

For many weeks none in the party had gazed upon aught but the white waste of snow and ice.

The green valley now seemed to partly blind them, and, indeed, it was some while before any could take in its appointments in full.

Then they saw that a small settlement of stone houses was near at hand.

Beyond was another, larger, and in the midst of it was one large building covering fully an acre.

It looked as if the Antarctic natives had expected the attack, for they were gathered about their huts with arms ready for battle.

At sight of the white men they set up a fearful yelling, and danced about, brandishing their weapons.

“They mean to give us a warm reception, don’t they?” cried Frank. “Now where do you suppose the prisoners are?”

“Probably in that large building,” said Hardy, with conviction; “that seems to be the stronghold of the tribe.”

“What shall we do? Make an open attack?” asked Jack Wallis.

“First let us see if we cannot treat with them,” said Frank.

But this was quickly proved out of the question.

The words had barely left his lips when there was a startling sound in his rear.

Instantly from behind rocks and shrubs a score of armed barbarians sprang forth and rushed upon our adventurers like an avalanche.

Swinging their battle axes they looked formidable indeed. The white men had barely time to prepare for defense, so sudden and swift was the murderous attack.