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, 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 of the entrance the doors would be lowered and the space filled with water and frozen. Then the waters would go on rising, the entrance would be submerged, and the defences of the fortress in which the remnant of humanity was to make its last stand for life would be complete. But in addition to these outer defences there was an enormous amount of work to be done in fitting the interior of the caverns to receive those for whom they were to form an asylum. They were already lighted by myriads of electric lamps, but the source of light was outside, and this had to be replaced by power-stations inside. Provision had to be made for keeping the air pure and vital, for supplying food and drink for an almost indefinite time, and for storing up a sufficiency of seeds and roots and treasures of art and creative skill, so that the new world might be clothed again with verdure and nothing essential of the splendid civilisation of Aeria be lost. Such, in the briefest outline, was the momentous task to which the Aerians devoted all their splendid genius and unconquerable energies, and day by day and week by week they toiled at it, while the fatal hour which was to witness the last agony of man upon earth swiftly drew nearer and nearer. The messages to the outside world had been sent and replied to. Those to the astronomers and to the governments of the Federation had been acknowledged in formal terms, which thinly concealed the incredulity with which they had been received. Olga had treated the message with the silent disdain of a conquering autocrat--such, as in sober truth, she now was. The Sultan had replied to it in a despatch in which the dignity of a victorious despot and the fatalism of the religious fanatic were characteristically blended. Then one by one the telephonic communications with the various parts of the world ceased; messages were sent out and repeated, but no answer came back. First Europe, then Britain, then South Africa, America, and Australia, ceased to respond to the signals; and by the beginning of July Aeria was completely isolated from the rest of the world--probably the only stronghold that now remained unsubdued by the conquering fleets of the Sultan and the Tsarina.