The star turtle who carries the Discworld on its back. Ten-thousand-mile-long member of the species Chelys galactica, and the only turtle ever to feature on the Hertzsprung-Russell Diagram. Almost as big as the Disc it carries. Sex unknown.

Shell frosted with frozen methane, pitted with meteor craters and scoured with asteroidal dust, its eyes are like ancient seas, crusted with rheum. Its brain is the size of a continent, through which thoughts move like glittering glaciers.

It is as large as worlds. As patient as a brick. Great A'Tuin is the only creature in the universe that knows exactly where it is going.

Upon its back stand Berilia, Tubul, Great T'Phon and Jerakeen, the four giant elephants upon whose shoulders the disc of the world rests. A tiny sun and moon spin around them on a complicated orbit to induce seasons, although probably nowhere else in the multiverse it is sometimes necessary for an elephant to cock its leg to allow the sun to go past.

After the events of The Light Fantastic, the Great A'Tuin was orbited by eight baby turtles, each with four small world-elephant calves and tiny discworlds, covered in smoke and volcanoes. They have subsequently begun their own cosmic journeys.

Wizards have tried to tune into Great A'Tuin's mind. They trained up on tortoises and giant sea turtles to get the hang of the Chelonian mind. But although they knew that the Great A'Tuin's mind would be big, they rather foolishly hadn't realised it would be slow. After thirty years all they found out was that the Great A'Tuin was looking forward to something.