"In a distant and second-hand set of dimensions, in an astral plane that was
never meant to fly, the curling star-mists waver and part ...
Great A'Tuin the turtle comes, swimming slowly through the interstellar gulf, hydrogen frost on his ponderous limbs, his huge and ancient shell pocked with meteor craters. Through sea-sized eyes that are crusted with rheum and asteroid dust He stares fixedly at the Destination.
In a brain bigger than a city, with geological slowness, He thinks only of the Weight.
Most of the weight is of course accounted for by Berilia, Tubul, Great T'Phon and Jerakeen, the four giant elephants upon whose broad and star-tanned shoulders the disc of the World rests, garlanded by the long waterfall at its vast circumference and domed by the baby-blue vault of Heaven.
Astropsychology has been, as yet, unable to establish what they think about.
The Great Turtle was a mere hypothesis until the day the small and secretive kingdom of Krull, whose rim-most mountains project out over the Rimfall, built a gantry and pulley arrangement at the tip of the most precipitous crag and lowered several observers over the Edge in a quartz-windowed brass vessel to peer through the mist veils.
The early astrozoologists, hauled back from their long dangle by enormous teams of slaves, were able to bring back much information about the shape and nature of A'Tuin and the elephants but this did not resolve fundamental questions about the nature and purpose of the universe.
For example, what was A'Tuin's actual sex? This vital question, said the Astrozoologists with mounting authority, would not be answered until a larger and more powerful gantry was constructed for a deep-space vessel. In the meantime they could only speculate about the revealed cosmos.
There was, for example, the theory that A'Tuin had come from nowhere and would continue at a uniform crawl, or steady gait, into nowhere, for all time. This theory was popular among academics.
An alternative, favoured by those of a religious persuasion, was that A'Tuin was crawling from the Birthplace to the Time of Mating, as were all the stars in the sky which were, obviously, also carried by giant turtles. When they arrived they would briefly and passionately mate, for the first and only time, and from that fiery union new turtles would be born to carry a new pattern of worlds. This was known as the Big Bang hypothesis."
- Terry Pratchett, The Colour of Magic