Right straight into the retreating violet beam they went, at top speed. They gained rapidly. The heat was furnace-like. The commander, watching the range-finder, kept moving the helm slightly over. A shaft of violet heat spanned the two shells of metal. For ten seconds it had held on the _NX-1_. The black dot of the enemy craft moved slowly to exact center on the dial. Fifteen seconds ... twenty ... twenty-three-- "Fire!" Graham jammed the torpedo lever back. "Crash dive!" The deck tilted downward. And Wells' white lips formed the words, "So long, Hemmy!"--and he tore the phones from his head. Seconds later a titanic explosion sounded through the cavern; echoed and re-echoed in vasty roars. The American craft's lights went off--but not before her men had seen, in the teleview, a fire-shot maelstrom where a moment before the octopi submarine had been. "We got them!" yelled Graham. * * * * * A roar of exultation burst from every throat. The men flung their arms out, jumped, yelled crazily. Faint emergency lights lit the scene. "Below, at regular posts," Wells ordered. "Reload bow and stern tubes. Graham, see to the lights." He himself remained at the helm. In a few moments the submarine had climbed back to the level of the tunnel. At quarter speed she nosed into the wide entrance, and slowly forged into the dense, deceptive shadows. The commander acted mechanically. Again by touch he steered his ship through the black, ragged cleft. Fifteen minutes after leaving the cavern of the octopi her bow poked through the weaving kelp into the free, salty depths of the Atlantic Ocean. There was one more task to perform, and Wells lost no time in doing it. When two hundred yards away he halted the _NX-1_, steadied her and sighted the stern tubes just above the dark tunnel hole. Quickly he sent forth two torpedoes. A huge roar rumbled through the water, whipping the beds of kelp to mad convulsions. "Turn around," the commander ordered harshly. He sighted his bow tubes and again let loose a bolt of two torpedoes. Then he sent the submarine forward, and, through the teleview, examined what his four weapons had done. Huge chunks of rock had been tumbled down, completely closing the tunnel. "Well," said Graham, "it's over! Finished! They'll never get through that!" * * * * * A full-throated cheer burst from the men below, a cheer that rang for minutes as they realized they were free forever of the octopi, of the cold underwater city, of the clutching tentacles. Graham grinned broadly. "Sound happy--eh?" he chuckled. "Say, Keith, it's good we've got those two octopi our fighting cook killed. Knapp would never believe our story without them!" He stared curiously at his commander. Wells was standing quite still, facing the teleview screen. A strange, far-away look was in his eyes.