Davis came to Terry, where he stood by the recording instrument. "The fish have gone," he said in a flat voice, "they've gone away. They didn't scatter. We'd have seen it. Do you realize where they went?" Terry nodded. "Straight down. Do you want to hear an impossible explanation?" "I've thought of several," said Davis. Doug came and picked up the gun-cameras that Terry and Deirdre had used and went away with them. "There's a kind of sound," said Terry, "that fish don't like. They won't go where it is. They try to get away from it." Deirdre said quietly, "I would too, if I were swimming." "Sound," said Terry, "in water as in air, can be reflected and directed, just as light can be. A megaphone turns out one's voice in a cone of noise, like a reflector on a light. It should be possible to project it. One can project a hollow cone of light. Why not a hollow cone of sound, in water?" Davis said with an unconvincingly ironic and skeptical air, "Indeed, why not?" "If such a thing were done," said Terry, "then when the cone of sound was turned on, the fish inside it would be captured as if by a conical net. They couldn't swim through the walls of sound. And then one can imagine the cone made smaller; the walls drawn closer together. The fish would be crowded together in what was increasingly like a vertical, conical net, but with walls of unbearable noise instead of cord. It would be as if the sea were electrified and the fish were shocked when they tried to pass a given spot." "Preposterous, of course," said Davis. But his tone was not at all unbelieving. "Then suppose something were sent up to the top of the cone, and it projected some kind of a cover of sound on the top of the cone and imprisoned the fish with a lid of sound they couldn't endure. And then suppose that thing sank into the water again. The fish couldn't swim through the walls of noise around them. They couldn't swim through the lid of sound above them. They'd have to swim downward, just as if a hood were closing on them from above." "Very neat," said Davis. "But of course you don't believe anything of the sort." "I can't imagine what would produce that sound in that way and send up a cork of sound to take the fish below. And I can't imagine why it would be done. So I can't say I believe it." Davis said slowly, "I think we begin to understand each other. We'll stay as close to this place as we can until dawn, when we will find nothing to show that anything out of the ordinary happened here." "Still less," said Terry, "to hint at its meaning. I've been doing sums in my head. That bright water was almost solid with fish. I'd say there was at least a pound of fish to every cubic foot of sea." "An underestimate," said Davis judicially. "When the bright patch was a thousand yards across--and it was even more--there'd have been four hundred tons of fish in the top three-foot layer."