Great A'Tuin is a turtle. So what if it has four World Elephants and a disc-shaped world on top of it? It’s still a turtle, of the species Chelys galactica. Nobody knows where it goes, or why, except probably Great A'Tuin itself.
A year ago astronomers across the Discworld had been puzzled to see the stars gently wheel across the sky as the world-turtle executed a roll. The thickness of the world never allowed them to see why, but Great A’Tuin’s ancient head had snaked out and down and had snapped right out of the sky the speeding asteroid that would, had it hit, have meant that no one would ever have needed to buy a diary ever again.
His name–or Her name, according to another school of thought–was Great A'tuin; he–or, as it might be, she–will not take a central role in what follows but it is vital to an understanding of the Disc that he–or she–is there, down below the mines and sea ooze and fake fossil bones put there by a Creator with nothing better to do than upset archaeologists and give them silly ideas.