Great A'Tuin, the World Turtle, of the species Chelys galactica, is the gigantic turtle upon whose back the Discworld was carried through space. The Disc does not rest directly on A'Tuin; instead, it rests on the shoulders of four immense elephants, Berilia, Tubul, Great T'Phon and Jerakeen, who stand atop the turtle's shell.
Many things remained unknown about Great A'Tuin, including its gender and desires. The scientists and philosophers send a space ship containing Rincewind off the edge of the Disc to try to discover the sex of the turtle but his is a failure. These were matters of constant speculation by philosophers, mystics and theologians. Nobody knows where it goes, or why, except probably Great A'Tuin itself.. It has been known to alter its course to avoid meteorite hits, and even to snap at them with its beak or send them away with its flippers.
It is believed that Discworld was originally meant to have five world-supporting elephants but that one of them crash landed on the Disc forming Fat Mountain (Schmaltzberg) in Uberwald.
Discword is not the only Discworld in the known universe since at the conclusion of The Light Fantastic, eight eggs hatch into eight baby Discworlds, which are no doubt travelling through Space and spreading the astrochelonian model of world formation, much to the disbelief and consternation of any astronomers looking upwards through telescopes from less favoured worlds. (Note that the baby turtles each had four baby elephants on board.)
The Light Fantastic might also offer an inferred clue to the gender of Great A'Tuin - just as we speak of Mother Earth, is it possible that this is a nurturing Mother Turtle, who popped back to see if the eggs were all right? In which case, either parthenogenesis applies, or the mating of two star turtles occurred, very very carefully (much more so than Discworld philosophers fear - although this may have been the trigger event causing the Fifth Elephant to lose its footing and fall off the turtle?), a long time ago, before the evolution of sentient life, and it's taken this long for the eggs to incubate in the light of the red sun.
Great A'Tuin takes his Roundworld origins from Hindu mythology. The "world-elephants" are mythical animals which appear in Hindu cosmology. The Amarakosha (5th century) lists the names of eight male elephants bearing the world (along with eight unnamed female elephants). The names listed are Airavata, Pundarika, Vamana, Kumunda, Anjana, Pushpa-danta, Sarva-bhauma, and Supratika. The names of four elephants supporting the earth from the four directions are given in the Ramayana : Viroopaaksha (east), Mahaapadma (south), Saumanasa (west), Bhadra (north).
Brewer's Dictionary of Phrase and Fable lists Maha-pudma and Chukwa are names from a "popular rendition of a Hindu myth in which the tortoise Chukwa supports the elephant Maha-pudma, which in turn supports the world". The spelling Mahapudma originates as a misprint of Mahapadma in Sri Aurobindo's 1921 retelling of a story of the Mahabharata,
The popular rendition of the World Turtle supporting one or several World Elephants is recorded in 1599 in a letter by Emanual de Veiga. Wilhelm von Humboldt claimed, without any proof, that the idea of a world-elephant maybe due to a confusion, caused by the Sanskrit noun Nāga having the dual meaning of "serpent" and "elephant" (named for its serpent-like trunk), thus representing a corrupted account of the world-serpent. In some legends the world is supported on a stack of elephants standing on the backs of each other and descending to infinity. In others, there is a mixture of animals standing on each other.