Since the laser drilling head sealed the walls of the hole with a coating of frozen lava, it was necessary to probe horizontally for samples. Small extension drills, capable of reaching five feet on either side of the hole, were carried in the head for this purpose. Jim lowered the head through the last twenty feet of its drilling limit. Every six inches he sent the horizontal probes to their limits. The tell-tale chemicals existed at every point. He computed the volume he had probed, and felt numb. By the time Sam had shown up, Jim had withdrawn the probe to the surface and was moving the Prospector slowly across the moon's surface. Sam saw the motion on the television screen. "Where are you going? I thought we were going to check out the hole we were in." "It's been checked," said Jim. He hesitated. His original plan had been to move the Prospector a distance of fifty feet and probe again to the five-hundred-foot level. Then, decisively, he pressed the control that kept the Prospector moving. He stopped it a hundred feet from the previous hole and began the long, tedious job of drilling again to the limits of the Prospector's equipment. * * * * * Sam spelled him off during the day. By evening, they had hit the four-hundred-and-fifty-foot level. Jim took his first analysis in this hole. The chemicals were there. In greater concentration than at the same level in the previous hole. Jim turned to Sam. "We have circuits for measuring potential differences on the lunar landscape. Could we make a reading at the bottom of this hole?" Sam considered. "It'll take some doing, but I think we can manage it. What do you expect to find from that?" Jim didn't dare tell him what was in his mind. "I don't know," he said. "But it might be worth trying--if there _is_ anything living down there--" By the following afternoon, Sam had made the necessary equipment arrangements so that potential readings could be obtained in the mass from which the chemical samples were being removed. The telemetered report was connected to a recorder that plotted the variations against a time scale. As soon as the circuit was set up and calibrated, the recording meter showed a response. A very slow, rhythmic pulsation showed in the inked line on the paper. Jim felt as if his breathing must have stopped for an infinite length of time. "That's what I thought we'd find," he said at last. "What?" said Sam. "I don't understand what you're talking about. What do you think those pulsations mean?" "Did you ever hear of an electroencephalograph?" said Jim, gravely. "Electro--Sure, brain wave recordings. Jim! You don't think these waves--!" In silence, the two men stared at the wavering pen and the sheet of recording paper that slowly unrolled beneath it. * * * * *