Something seized his flailing arm and pulled him up. The arm ceased to experience the horrible sensation of being in boiling oil. His hand recognized a gunwale. He swarmed up the solid object with hands helping him, and found himself in the boat, gasping and shivering, and cringing at the bare memory of the suffering he'd undergone. Deirdre stared at him, frightened. She swung the boat's bow shoreward. The outboard motor roared, and the boat raced past the gap in the reef and rushed toward the lagoon opening. "Are you all right? What happened? You were swimming and suddenly...." He swallowed. His hands quivered. He shook his head and then said unsteadily, "I meant to ... check the reason those queer fish stay in the lagoon. I thought that if they belonged in the depths and were somehow carried out of them, they would try to get back. I found out!" He felt an unreasonable relief when the lagoon entrance was behind the boat. The glassy water was reassuring. The _Esperance_ looked like safety itself. "I think I know how they got here, now," he added. "We underestimated what we're trying to understand. I'll be all right in a minute." It was less than a minute before he shook himself and managed to grin wryly at Deirdre. "Was there a hum in the water?" asked Deirdre, still staring at him. "I thought I heard it on the bottom of the boat. Was that the trouble?" "Yes. I wouldn't call it a hum," Terry admitted. "Not any longer. Now I know what a slow fire feels like." "You frightened me," said Deirdre, "the way you splashed...." "I heard the humming sound," said Terry, "last night when the yacht came up to the island. We were perhaps a half-mile off-shore. It was very faint, but I had the amplifier turned down low. The hum was at its loudest just before we passed the reef, but nobody else noticed. When Dr. Morton said there were abyssal fish in the lagoon, I knew why they'd be there. I made a guess at what might drive them there. I went to find out if I was right. I found out!" "The hum?" asked Deirdre again. When he nodded, she said: "What are you going to do now? What do you think makes the hum?" "I'm trying hard not to guess what makes the hum," Terry told her. "Insufficient data. I need more. I think I'll ask what other odd phenomena have turned up in this neighborhood. Foam-patches on the sea? I can't imagine a connection, but still...." He swung the little boat alongside the docked _Esperance_ and held out his hand to help Deirdre to the dock. His hand was wholly steady again. She accepted the help. "We'll go to the tracking station?" "Yes. Everybody seems to be there," said Terry.