Terry and Deirdre went to look for a place where they could talk over something privately. It was of enormous importance to them, but it was not connected with fish or meteorites or plastic objects or anything at all but the two of them. And to them the yacht seemed crowded with people, even though there was nobody else abovedecks but one of the crew-cuts at the wheel. When the _Esperance_ entered the lagoon the next morning, though, their private talk had evidently come to a satisfactory conclusion. Deirdre smiled at Terry without any reason whatever, and he looked at once smug and embarrassed and uneasy, as if he possessed a new status to which he was still unaccustomed. The recorder, trailing a submarine ear overboard, had duly reported the presence of the hum in the water, just outside the lagoon. It had not been operating for forty hours or thereabouts. During that time the fish inside could go out of the lagoon, if they chose. And other fish could come in. Terry said suddenly, as the yacht went under power toward the tracking station wharf, "Suppose there was a cone of noise just outside the lagoon, and the flanks of the submarine mountain under us were included in the cone? And suppose the cone grew smaller, like the other one. What would happen?" Deirdre shook her head, smiling at him. "The fish," said Terry, "could escape into the lagoon." "Probably," agreed Deirdre. "And if fish could be driven downward along a certain path," said Terry, "the way we saw it happen, why, fish could be driven up in a certain path, too." "Obviously," said Deirdre. "So if something wanted to replace the fish in the lagoon, or to add to their number, why, it would puncture their swim bladders far, far down, and then drive them up to the surface and into the lagoon, and then keep the noise going to keep them inside." "Is this a new idea?" asked Deirdre. "N-n-o," admitted Terry. "I've had it for some time." "So," said Deirdre, "have I." The _Esperance's_ engine stopped, and she floated to gentle contact with the wharf. Members of the tracking station staff made the yacht fast. With others, Dr. Morton came on board. His expression was the picture of unrelieved gloom. "I'm in a nice spot!" he told Davis. "I predicted a second bolide correctly! I had to use a different retardation factor to make the math come out right. Now I'm asked to explain that! How can I tell them I knew where it would fall, and only had to compute when?" "Come below and look at the pictures we got," said Davis. They disappeared down the after-cabin hatch. Terry knew about the pictures. Doug had developed them with sweating care, developing each negative separately and adjusting the development-time to the varying exposures of the bright object.