"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... See... 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, wit 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."
This is the prologue of "The Colour of Magic", the first of twenty-two Discworld novels by Terry Pratchett. Discworld is a disc-shaped world really transported through space on the shoulders of four giant elephants mounted on the back of a giant turtle which sometimes snaps at a passing comet.
The idea of a giant turtle crawling in the space is not always accepted on Discworld. For instance, there exists some religions who think that the world is in fact a big sphere spinning around an enormous ball of burning gas. This seems so evident that they usually prefer to kill anyone who claims that the universe is not as simple as it appears to the mind. "And yet the Turtle does move".
Since a kingdom whose mountains project out over the Rimfall build a machine to peer under the Disc, the existence of Great A'Tuin and his Elephants is a certitude. But some fundamental questions about the nature of the universe remain unresolved. For instance: "What do the elephants think about ?" or, more important, "What is A'Tuin's actual sex ?". (And one can conjecture the impact on Discworld if the Turtle is a She and if She meets a male world carrier).