WHEN the news of what had happened at midnight in the palm grove was published the next morning far and wide through the valley of Aeria it would have been impossible to imagine that an irrevocable sentence of death was overhanging the land and all its inhabitants, save those who were to be selected to take the one chance that remained of surviving the chaos that was to come.

There was no one in the valley to whom Alan’s story was not familiar in all its details, there was not a single heart that had not in the midst of its own happiness sympathised with him and Alma in their sorrow, and so, when that sorrow was at last turned into joy, everyone forgot for the moment the fate whose approach was so near and so certain, and rejoiced with them in the happiness that was great enough to raise them above the gloom that was already stealing over the world.

But in the midst of the general rejoicing came the decision of the Council upon the request which Alan had submitted to his father, and this, though he was forced to confess it wise and just, was by no means what, in his enthusiasm, he could have wished. The rulers of Aeria absolutely refused to permit any of the air-ships to leave the valley for at least two months to come.

They recognised with perfect approval the nobility of the resolve which Alan had taken to carry the message of the[320] world’s approaching end to those nations which he had been, partially at least, responsible for plunging into the horrors of war, but they insisted that the concerns of Aeria must, in their eyes, take precedence of those of the outside world.

There was much to do, and the time for doing it was short. What was perhaps the greatest engineering task in the history of the world had to be conceived and completed within the next four months, and as Alan and Alexis were admittedly the two most skilful practical engineers in the State, the Council declined to allow them to run the almost certain risk of death at the hands of their enemies when their knowledge and skill ought to be devoted to the work of ensuring, as far as possible, the preservation of that remnant of the human race who should be destined to seek safety in the caverns of Mount Austral.

When the completion of that work was made certain, then permission would be freely given to them and their companions to go forth and proclaim their warning to the world, subject only to the condition that they were to take every precaution consistent with the honour of their race to return while there was yet time for them to take their places among the Children of Deliverance should the selection fall upon them.

Meanwhile, telephonic messages were to be sent to all those portions of the world with which Aeria was still in communication, conveying the exact terms of the warning that had been received from Mars, and calling upon the astronomers in all the observatories on the globe to verify the calculations for themselves, and publish their conclusions to their respective nations as quickly as possible.

With these terms Alan was of necessity obliged to be content. Indeed, when he came to review them in sober thought, he saw that, while nothing was to be lost, much was to be gained by submission to them.

Though he still refused, even in spite of the knowledge that he would share with Alma the future if there was to be one, to obey the order of the Council which exempted him from the ordeal of selection, he thought and worked with just as[321] much ardour as though the safety of the whole of the dwellers in Aeria, as well as his own, hung upon his efforts.

The caverns of Mount Austral, like those of other limestone formations in various parts of the world, had been formed in some remote geological period by the solvent action of water charged with carbonic gas upon the limestone rocks.

The entrance to them, discovered very soon after the valley had been colonised by the Terrorists in the first decade of the twentieth century, was situated on the inner slopes of the mountain about eight hundred feet above the level of the lake, which occupied the central portion of the valley.

This lake, although fed by hundreds of streams from the surrounding mountains, always preserved the same level, in spite of the fact that it had no visible outlet. Those who first explored the caverns found the explanation of this phenomenon.

Below the floors of the vast chambers which penetrated the heart of the mountain for a distance of nearly three miles there ran a deep chasm, through which rushed in a black, swift, silent stream the surplus waters of the lake. This stream was nearly a thousand feet below the entrance to the caverns and half that distance below the floor of the lowest chambers and galleries.

The scheme conceived by Alan and Alexis and their fellow-workers was in fact nothing less than the damming of this subterranean stream by a mighty sluice-gate composed of one huge sheet of metal which, running down into grooves cut in the solid rock and metal-sheathed, should completely close the inner mouth of the tunnel by which the waters entered the caverns.

This, once successfully fixed in its place, would deprive the lake of its only known outlet. The streams would go on flowing from the mountains and the waters of the lake would rise. The upper entrance would, when the fatal moment came, also be closed, not by one such door, but by three that would slide down one behind the other in the upper tunnel, which,[322] with a diameter of about thirty feet and a height of almost fifty, ran for nearly a quarter of a mile from the side of the mountain to the first of the chambers.

The spaces between these doors would be filled with ice artificially frozen, and shafts to allow for expansion should the ice melt and the water boil would run from them vertically, piercing the mountain-side. When the waters rose to the level............