They found a surprising simplicity in the arrangements underground. Two rooms only had been carved from the solid rock, and one of these ended in a wall of gray metal that could be only the great base of the gun. But nowhere was a complication of mechanism that might be damaged or destroyed, nor any wiring or firing device. A round door showed sharp edges in the gray metal, but only the strength of many men could have removed its huge bolts, and these two knew there must be other doors to seal in the mighty charge. "Not a wire!" the scientist exclaimed. "How do they fire it?" The answer came to him with the question. "Radio, of course; and the receiving set is in the charge itself; the barrel of the gun is its own antenna. They must fire it from a distance--back on the island where we were, perhaps. It would need to be accurately timed." "Come on!" shouted McGuire, and raised the flask of explosive to his shoulder. * * * * * Each one knew the need for haste; each waited every moment for the terrible blast of gun-fire that would jar their bodies to a lifeless pulp or, by detonating their own explosive, destroy them utterly. But they carried the flasks again to the top, and the three of them worked breathlessly to place their whole supply where McGuire directed. The massive barrel of the gun was beside them; it was held in tremendous castings of metal that bolted to anchorage in the ground. One great brace had an overhanging flange; the explosive was placed beneath it. Professor Sykes had come prepared. He attached a detonator to one of the flasks, and while the other two were placing the explosive in position he fastened two wires to the apparatus with steady but hurrying fingers; then at full speed he ran with the spool from which the wires unwound. McGuire and Althora were behind him, running for the questionable safety of the sand-hills. Sykes stopped in the shelter of a tiny valley where winds had heaped the sand. "Down!" he shouted. "Get down--behind that sand dune, there!" He dropped beside them, the bared ends of the wires in his hands. There was a battery, too, a case no larger than his hands. Professor Sykes, it appeared, had gained some few concessions from his friends, who had learned to respect him in the field of science. One breathless moment he waited; then-- "Now!" he whispered, and touched the battery's terminals with the bare wires. * * * * * To McGuire it seemed, in that instant of shattering chaos, that the great gun itself must have fired. He had known the jar of heavy artillery at close range; he had had experience with explosives. He had even been near when a government arsenal had thrown the countryside into a hell of jarring, ear-splitting pandemonium. But the concussion that shook the earth under him now was like nothing he had known.