Great A'Tuin is the fictional giant star turtle in the Discworld universe, who travels through space, carrying the four giant elephants (named Berilia, Tubul, Great T'Phon, and Jerakeen) who in turn carry the Discworld. A member of the species Chelys galactica, A'Tuin is the only turtle to ever feature on the Hertzsprung-Russell diagram. Its shell is frosted with frozen methane, pitted with meteor craters, and scoured with asteroidal dust.

Great A'Tuin's sex is unknown, but is the subject of much speculation by some of the Disc's finest scientific minds - in an analogy to astrophysicists, specialists in this field are called astrochelonians.The sex of The World Turtle is pivotal in proving or disproving a number of conflicting theories about the destination of "Great A'Tuin's" journey through the cosmos. If (as one popular theory states) "Great A'Tuin" is moving to his (or her) mating grounds, then at the point of mating might the civilisations of the Disc be crushed or simply slide off? Attempts by telepaths to learn more about Great A'Tuin's intents have not met with much success, mainly because they did not realise that its brain functions on such a slow timescale.

Following the events in The Light Fantastic, Great A'Tuin attended the hatching of eight baby turtles, each with four baby elephants and a tiny Discworld of their own. They have since gone off on their own journeys.

The gender of the Turtle is something of a mystery to the inhabitants of the disc, being not able to peer underneath the turtle themselves. This leads to some great debate and argument as to what position the turtle would assume should there be another cosmic big bang.

See also

• Turtles all the way down