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

Diaspar that lay below them.
    “That is no model; it does not really exist. It is merely the projected image of the pattern held in the Memory Banks, and therefore it is absolutely identical with the city itself. These viewing machines here enable one to magnify any desired portion, to look at it life size or larger. They are used when it is necessary to make alterations in the design, though it is a very long time since that was done. If you want to know what Diaspar is like, this is the place to come. You can learn more here in a few days than you would in a lifetime of actual exploring.”
    “It’s wonderful,” said Alvin. “How many people know that it exists?”
    “Oh, a good many, but it seldom concerns them. The Council comes down here from time to time; no alterations to the city can be made unless they are all here. And not even then, if the Central Computer doesn’t approve of the proposed change. I doubt if this room is visited more than two or three times a year.”
    Alvin wanted to know how Khedron had access to it, and then remembered that many of his more elaborate jests must have involved a knowledge of the city’s inner mechanisms that could have come only from very profound study. It must be one of the Jester’s privileges to go anywhere and learn anything; he could have no better guide to the secrets of Diaspar.
    “What you are looking for may not exist,” said Khedron, “but if it does, this is where you will find it. Let me show you how to operate the monitors.”
    For the next hour Alvin sat before one of the vision screens, learning to use the controls. He could select at will any point in the city, and examine it with any degree of magnification. Streets and towers and walls and moving ways flashed across the screen as he changed the co-ordinates; it was as though he was an all-seeing, disembodied spirit that could move effortlessly over the whole of Diaspar, unhindered by any physical obstructions.
    Yet it was not, in reality, Diaspar that he was examining. He was moving through the memory cells, looking at the dream image of the city— the dream that had had the power to hold the real Diaspar untouched by time for a billion years. He could see only that part of the city which was permanent; the people who walked its streets were no part of this frozen image. For his purpose, that did not matter. His concern now was purely with the creation of stone and metal in which he was imprisoned, and not those who shared— however willingly— his confinement.
    He searched for and presently found the Tower of Loranne, and moved swiftly through the corridors and passageways which he had already explored in reality. As the image of the stone grille expanded before his eyes, he could almost feel the cold wind that had blown ceaselessly through it for perhaps half the entire history of mankind, and that was blowing now. He came up to the grille, looked out— and saw nothing. For a moment the shock was so great that he almost doubted his own memory; had his vision of the desert been nothing more than a dream?
    Then he remembered the truth. The desert was no part of Diaspar, and therefore no image of it existed in the phantom world he was exploring. Anything might lie beyond that grille in reality; this monitor screen could never show it.
    Yet it could show him something that no living man had ever seen. Alvin advanced his viewpoint through the grille, out into the nothingness beyond the city. He turned the control which altered the direction of vision, so that he looked backward along the way that he had come. And there behind him lay Diaspar— seen from the outside.
    To the computers, the memory circuits, and all the multitudinous mechanisms that created the image at which Alvin was looking, it was merely a simple problem of perspective. They “knew” the form of the city; therefore they could show it as it would appear from the outside. Yet even though he could appreciate how the trick was done,