A helicopter came rattling into view from the south. It grew in size and loudness, and presently hovered over the tracking station. Then it made a wide, deliberate circuit of the lagoon. At the inlet where the squid lay almost entirely in the water--but fastened by ropes lest it drift away--above that spot, the helicopter hovered for a long time. It must have been taking photographs. Presently, it lowered one man by a line to the ground. Obviously, the man could not endure any delay in getting at so desirable a biological specimen. Then the helicopter went droning and rattling to the tracking station, and landed with an air of weariness. _La Rubia_ continued to try to catch fish. They were here in plenty. But the coral heads were everywhere. Nets tore. Ropes parted. Capitán Saavedra waved his arms and swore. The _Esperance_ rumbled and circled away from the wharf, and headed for the lagoon entrance. The singular contrivance built during the night was in place at her bow. She passed _La Rubia_, on whose deck men frantically mended nets. The _Esperance_ passed between the small capes and the first of the ocean swells raised her bow and rocked her. She proceeded beyond the reef. The bottom of the sea dropped out of sight. Terry switched on the submarine ear and listened. The humming sound was to be expected here. It had stopped. It was present yesterday, and even during the night, when _La Rubia_ came into the lagoon. But now the sea held no sound other than the multitudinous random noises of fish and the washing, roaring, booming of the surf. Deirdre was aboard, of course. She watched Terry's face. He turned to the new instrument, and then dropped his hand. "I think," he said carefully to Davis, "that I'd like to make a sort of sweep out to sea. It's just possible we'll find the hum farther out." Deirdre said quickly, "I think I know what you're up to. You want to survey a large area of the ocean while something comes up. Then you can direct that "something" to the lagoon mouth by using your sound device, so the ... whatever-it-is has to take refuge in the lagoon. Since we've killed the squid...." "That's it," said Terry. "Something like that happened when we speared the fish. The squid took their place. Now we've killed the squid. Just possibly...." They found the humming sound in the water four miles off-shore. They traced it through part of a circle. If something were being driven upward, it could not pass through that wall of humming sound. "That proves your point," Davis said. "Now what?" Without realizing it, he'd yielded direction of the enterprise to Terry, who had unconsciously assumed it. "Let's go back to the island," said Terry thoughtfully. "I've got a crazy idea--really crazy! I want to be where we can duck into shallow water when we try the new projector."