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.
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.
In case anyone wants a cheaply made animated .gif of Great A'Tuin doing the Disco-World dance, without all the text from the original post: here you go.
(On the other hand, if you’re looking for Discworld FILK versions of David Bowie songs, especially ones about Sam Vimes’ thin-soled boogie booties, you should take a moment to read through the original post.)
“There was no analogy for the way in which Great A'Tuin the world turtle moved against the galactic night.
When you are ten thousand miles long, your shell pocked with meteor craters and frosted with comet ice, there is absolutely nothing you can realistically be like except yourself.
So Great A'Tuin swam slowly through the interstellar deeps like the largest turtle there has ever been, carrying on its carapace the four huge elephants that bore on their backs the vast, glittering waterfall-fringed circle of the Discworld, which exists either because of some impossible blip on the curve of probability or because the gods enjoy a joke as much as anyone.”