We enter a "distant and secondhand set of dimensions" (1.Prologue.1), and see the Great A'Tuin continue his timeless journey swimming through the cosmos
On his back are four elephants whose shoulders bear the weight of the Discworld.
As obvious as he may seem to us, A'Tuin was merely a hypothesis for the Disc's people until the kingdom of Krull built a gantry to lower observers over the world's edge. In that moment, a major mystery was solved and the field of astrozoology began taking applications.
But the answers only lead to more questions, such as where was A'Tuin headed and, for that matter, why?
So it is that a young cosmochelonian is testing out his new telescope in the hopes of measuring the albedo of A'Tuin's eye, when he spies smoke rising from the old city in the world.