With undiminished velocity they were flashing downward in a long slant toward the beleaguered Third City, and from the flying vessel there was launched toward the city's central lagoon a torpedo. No missile this, but a capsule containing a full ton of allotropic iron, which would be of more use to the Nevian defenders than millions of men. For the Third City was sore pressed indeed. Around it was one unbroken ring of boiling, exploding water--water billowing upward with searing, blinding bursts of superheated steam, or being hurled bodily in all directions in solid masses by the cataclysmic forces being released by the embattled fishes of the greater deeps. Her outer defenses were already down, and even as the Terrestrials stared in amazement another of the immense hexagonal buildings burst into fragments; its upper structure flying wildly into scrap metal, its lower half subsiding drunkenly below the surface of the boiling sea. The three Terrestrials involuntarily seized whatever supports were at hand as the Nevian space-ship struck the water with undiminished speed, but the precaution was needless--Nerado knew thoroughly his vessel, its strength and its capabilities. There was a mighty splash, but that was all. The artificial gravity was unchanged by the impact; to the passengers the vessel was still motionless and on even keel as, now a submarine, she snapped around like a very fish and attacked the rear of the nearest fortress. For fortresses they were; vast structures of green metal, plowing forward implacably upon immense caterpillar treads. And as they crawled they destroyed, and Costigan, exploring the strange submarine with his visiray beam, watched and marveled. For the fortresses were full of water; water artificially cooled and aerated, entirely separate from the boiling flood through which they moved. They were manned by fish some five feet in length. Fish with huge, goggling eyes; fish plentifully equipped with long, armlike tentacles; fish poised before control panels or darting about intent upon their various duties. Fish with intelligent brains, waging desperate war upon a hated foe! Nor was their warfare ineffectual. Their heat-rays boiled the water for hundreds of yards before them and their torpedoes were exploding against the Nevian defenses in one appallingly continuous concussion. But most potent of all was a weapon unknown to Triplanetary warfare. From a fortress there would shoot out, with the speed of a meteor, a long, jointed, telescopic rod, tipped with a tiny, brilliantly shining ball. Whenever this glowing tip encountered any obstacle, that obstacle disappeared in an explosion world-wracking in its intensity. Then what was left of the rod, dark now, would be retracted into the fortress--only to emerge again in a moment with a tip once more shining and potent.