Nerado, apparently as unfamiliar with the peculiar weapon as were the Terrestrials, attacked cautiously; sending out far to the fore his murkily impenetrable screens of red. But the submarine was entirely non-ferrous, and its officers were apparently quite familiar with the Nevian beams which licked at and clung to the green walls in impotent fury. Through the red veil came stabbing tiny ball after brilliant ball, and only the most frantic dodging saved the space-ship from destruction in those first few furious seconds. And now the Nevian defenders of the Third City had secured and were employing the vast store of allotropic iron so opportunely delivered by Nerado. From the city there pushed out immense nets of metal, extending from the surface of the ocean to its bottom; nets radiating such terrific forces that the very water itself was beaten back and stood motionless in vertical, glassy walls. Torpedoes were futile against that wall of energy. The most fiercely driven rays of the fishes flamed incandescent against it, in vain. Even the incredible violence of a concentration of every available force-ball against one point could not break through. At that unimaginable explosion water was hurled for miles. The bed of the ocean was not only exposed, but in it there was blown a crater at whose dimensions the Terrestrials dared not even guess. The crawling fortresses themselves were thrown backward violently and the very world was rocked to its core by the concussion, but that iron-driven wall held. The massive nets swayed and gave back, and tidal waves hurled their mountainously destructive masses through the Third City, but the mighty barrier remained intact. And Nerado, still attacking two of the powerful tanks with his every weapon, was still dodging those flashing balls charged with the quintessence of destruction. The fishes could not see through the sub-ethereal veil, but all the rod-gunners of the two fortresses were combing it thoroughly with ever-lengthening, ever-thrusting rods, in a desperate attempt to wipe out the new and apparently all-powerful Nevian submarine, whose sheer power was slowly but inexorably crushing even their gigantic walls. "Well, I think that right now's the best chance we'll ever have of doing something for ourselves." Costigan turned away from the absorbing scenes pictured upon the visiplate and faced his two companions. "But what can we possibly do?" asked Clio, and "Whatever it is, we'll try it!" Bradley exclaimed.