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 geologist, and had spent much time in examining the strata of the region.

now, as they strolled along, he cast his gaze upward critically along the great rock wall. suddenly he came to a halt.

“frank,” he said, “i think i have hit upon a discovery.”

“eh!” exclaimed the young inventor.

“it is true, and if my theories prove correct, it is a most important one.”

“what is it?”

randall pointed up to several distinct lines of various heights on the mountain wall.

“do you see those lines?”

“yes.”

“the highest one is fully two hundred feet.”

“that is true.”

“well, have you never seen lines like those before?”

frank studied the face of the cliff a moment.

then he said:

“yes, i think i have. they look like high water marks upon cliffs at the seashore.”

“just so. now, if they are water marks, it must mean that there have been times when this whole basin, this entire polar country, has been under water.”

frank was astounded.

“at the glacial period?” he asked.

“glacial period be hanged! within a hundred years, more or less.”

“you don’t mean it?”

the two men gazed at each other. frank looked incredulous, but randall was convinced.

“mark you,” resumed the geologist, “i have closely examined the drift and strata of this region. all point to this conclusion. also that the basin has been occupied by water at different intervals. what i mean, is that the presence of water has been periodical.”

frank rubbed his eyes.

“in that case——”

“the place may become submerged again, and i believe that the period is not far distant. if my hypothesis is correct,” continued the geologist, “we have a very logical explanation of the abandonment of this country by its inhabitants.”

frank was so overcome by the astounding force of this declaration that for a time he could not speak.

after some thought he said:

“you have certainly hit upon a logical idea, randall. but if it is true, where does this flood come from, and how would the people know it?”

randall pointed to the distant column of smoke rising from the volcano.

“do you see that?” he asked. “it means that this entire region is governed by volcanic forces. now, the action of the internal forces, of which we know little, may be capable of bringing a vast volume of water periodically to the surface from subterranean basins. the pressure would be sufficient. synonymous with certain actions of yonder volcano, this beautiful land of promise is flooded to the brim.”

frank gazed keenly at randall. he had not given him credit for so much penetration.

“and that is why this country has been abandoned?”

“just so! it is easy to see how the people could tell when danger threatened. the eruptions of the volcano are doubtless periodical. the polar people knew just when to abandon the valley.”

“whew!” exclaimed frank. “then, according to that, it is apt to become flooded at any time now!”

“just so.”

“randall, you are keen.”

“pshaw! it only requires a little study. do you see that little rivulet trickling out from under the mountain wall?”

“yes.”

“well, that was not there yesterday.”

“eh?”

“it is true!”

even as he spoke, randall gave an exclamation. he pointed to a patch of turf near, and whispered:

“look—look! you cannot want better evidence.”

frank gazed in the direction indicated, and both beheld a most astounding thing.

the little patch of turf had begun to throb and heave. soon dew-like moisture was seen on the blades.

then up shot a little bulb of boiling water. it momentarily grew larger.

the turf was gently thrust aside and disintegrated, while a tiny stream flowed away down the incline, making its own course and momentarily growing larger.

a spring had burst into life in that moment!

it was wonderful!

“that is only one of many,” declared randall. “you shall see.”

deep in the center of the polar valley was a lake.

it was true that this was steadily rising above its banks. all this was prima facie evidence.

astonished, frank watched the phenomenon.

then he turned and swept a glance up at the mountain wall.

“it seems to me that our position, then, is one of peril.” he said. “what is to save us if the valley fills as you aver? we would be drowned like rats in a trap.”