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 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.