"I know," he said. He could not make out any detail, but there they were, just as they had been for--how long? Ten thousand years? A hundred thousand? Five towers jutting straight up from the bleak plain, clustered around a central, taller tower. "Is the machinery still running?" he asked, pointlessly. "Of course." "There was some talk a year or so back about trying to stop it." She shook her head. "They wouldn't dare." * * * * * The machine had been discovered more than ten years earlier, when the first Earthmen landed on Titan. Saturn's largest satellite was devoid of life, a world of dark and cold, of hydrogen atmosphere and methane clouds, of ammonia seas and ice mountains. And there in the midst of it all stood the machine: a brazenly unconcealed cluster of mammoth buildings, with its five stately towers surmounted by the soaring central sixth. And within, row upon row of unexplained machinery, fully automated, operating continuously in perfect order. Alien. The discoverers soon concluded that the machine was unbelievably old, older than the Egyptian pyramids, perhaps even older than the Martian canals. And it was running smoothly. For untold centuries, for uncounted millennia, it had continued to operate efficiently, tended only by automatic machines. A clear challenge to the space-rovers from Earth. Who made this machine? How does it work? Why is it here? What is it doing? As soon as its discovery was made known, the machine was visited by a steady stream of Earthmen--physicists, archeologists, engineers of a thousand different specialties, and soldiers, politicians, men who were now forced to believe the inevitable. The machine was photographed, x-rayed, blueprinted, analyzed spectroscopically, philosophically, even theologically. Who built it? How does it work? Why is it here? No answers. Dr. Sidney Lee, an anthropologist who had made a name for himself by unraveling the history of the ruins on Mars, arrived on Titan full of optimism and enthusiasm. Twenty months later he was taken from Titan to a psychomedical center on Earth--completely irrational and suffering from man's oldest dread: the unknown. * * * * * Returning to the underground center that had grown over the years near the machine, to house the living and working quarters of the tiny scientific community on Titan, was something like returning home for Dr. Lee. Someone had seen to it that he got his old quarters back again. Most of the people he had known from five years ago had gone elsewhere, but a few remained. * * * * * Lee spent his first few days renewing acquaintances and meeting the new men and women. He was surprised at their youth, until he tried to recall how he must have looked and acted when he first arrived on Titan.