Buildings, an enormous number and variety of them, huge, not one of them less than five stories high, all with ramps instead of stairs. This was to be expected, considering the great size of the people who had lived there, and it followed the familiar pattern. But there was nothing in those buildings! On this airless world, there was no decay, no rust or corrosion--_and nothing to decay or corrode_. No pictures, tools, nothing that resembled sculpture, and while there were places where machines had stood, none were there now. Here and there in inaccessible locations were featureless blobs of metal. The implication was clear: Where they hadn't been able to remove a machine, they had melted in down on the spot. The thoroughness was bewildering. It wasn't done by some enemy; he would have stood off and razed the cities. But there was no rubble and the buildings were empty. The inhabitants themselves had removed all that was worth taking along. A whole people had packed and moved away, leaving behind only massive, echoing structures. There was plenty to learn, but nothing to learn it from. Buildings can indicate only so much and then there must be something else--at least some of the complex artifacts of a civilization--and there was none. Outside the cities, on the plains, there were the remains of plants and animals that indicated by their condition that airlessness had come suddenly. Sam Halden, the biologist, had examined them, but he discovered no clues. The unknown ancestor was still a mystery. And the others--Emmer, the archeologist, and Meredith, the linguist--had nothing to work on, though they searched. It was Kelburn who found the first hint. Having no specific task, now that the planet was located, he wandered around in a scout ship. On the other side of the planet, he signaled that there was a machine and that it was intact! The crew was hurriedly recalled, the equipment brought back into the ship, and they took off for the plain where Kelburn waited. And there was the machine, immense, like everything on the planet. It stood alone, tapering toward the sky. At the base was a door, which, when open, was big enough to permit a spaceship to enter easily--only it was closed. Kelburn stood beside the towering entrance, a tiny figure in a spacesuit. He gazed up at it as the three came near. "All we have to do is open it," he said. "How?" asked Meredith. She seemed to have forgotten that she disliked him. He had made a chance discovery because he had nothing to do while the others were busy, but she regarded it as further proof of his superiority. * * * * * It was hard to watch the happiness that her face directed toward Kelburn. Halden turned away. "Just press the button," he said. Emmer noticed his expression. "It's such a big button," he objected. "It's going to be hard to know when we find it."