The City and the Stars / The Sands of Mars by Arthur C. Clarke

rate of a thousand years a second. Already, we’re looking at the Diaspar of half a million years ago. We’ll have to go much farther back than that to see any real changes— I’ll increase the rate.”
    He turned back to the control board, and even as he did so, not one building but a whole block whipped out of existence and was replaced by a large oval amphitheater.
    “Ah, the Arena!” said Khedron. “I can remember the fuss when we decided to get rid of that. It was hardly ever used, but a great many people felt sentimental about it.”
    The monitor was now recalling its memories at a far higher rate; the image of Diaspar was receding into the past at millions of years a minute, and changes were occurring so rapidly that the eye could not keep up with them. Alvin noticed that the alterations to the city appeared to come in cycles; there would be a long period of stasis, then a whole rash of rebuilding would break out, followed by another pause. It was almost as if Diaspar were a living organism, which had to regain its strength after each explosion of growth.
    Through all these changes, the basic design of the city had not altered. Buildings came and went, but the pattern of streets seemed eternal, and the park remained as the green heart of Diaspar. Alvin wondered how far back the monitor could go. Could it return to the founding of the city, and pass through the veil that sundered known history from the myths and legends of the Dawn?
    Already they had gone five hundred million years into the past. Outside the walls of Diaspar, beyond the knowledge of the monitors, it would be a different Earth. Perhaps there might be oceans and forests, even other cities which Man had not yet deserted in the long retreat to his final home.
    The minutes drifted past, each minute an aeon in the little universe of the monitors. Soon, thought Alvin, the earliest of all these stored memories must be reached and the regression would end. But fascinating though this lesson was, he did not see how it could help him to escape from the city as it was here and now.
    With a sudden, soundless implosion, Diaspar contracted to a fraction of its former size. The park vanished; the boundary wall of linked, titanic towers instantly evaporated. This city was open to the world, for the radial roads stretched out to the limits of the monitor image without obstruction. Here was Diaspar as it had been before the great change came upon mankind.
    “We can go no farther,” said Khedron, pointing to the monitor screen. On it had appeared the words: REGRESSION CONCLUDED. “This must be the earliest version of the city that has been preserved in the memory cells. Before that, I doubt if the eternity circuits were used, and the buildings were allowed to wear out naturally.”
    For a long time, Alvin stared at this model of the ancient city. He thought of the traffic those roads had borne, as men came and went freely to all the corners of the world— and to other worlds as well. Those men were his ancestors; he felt a closer kinship to them than to the people who now shared his life. He wished that he could see them and share their thoughts, as they moved through the streets of that billion-year-remote Diaspar. Yet those thoughts could not have been happy ones, for they must have been living then beneath the shadow of the Invaders. In a few more centuries, they were to turn their faces from the glory they had won and build a wall against the Universe.
    Khedron ran the monitor backward and forward a dozen times through the brief period of history that had wrought the transformation. The change from a small open city to a much larger closed one had taken little more than a thousand years. In that time, the machines that had served Diaspar so faithfully must have been designed and built, and the knowledge that would enable them to carry out their tasks had been fed into their memory circuits. Into the memory circuits, also, must have gone the essential patterns