Great A'Tuin is a TURTLE, thank you very much. Tortoises cannot swim, while the Star Turtle is constantly swimming. The only creature in the multiverse who knows where it is going, G-A is the subject of much conjecture on the surface of Discworld. The Omnian Church maintained for a long time that the world was a sphere, despite the obvious logical flaw (All the water would fall off). Didactylos, an Ephebian philosopher, published a book called De Chelonian Mobilis (The Turtle Moves, for those who don't understand italics) debunking this theory. This precipitated a war, detailed further in Pratchett's Small Gods.