The Great A'Tuin's Shell is a massive, cosmic turtle that supports the Discworld, a flat planet in Terry Pratchett's fantasy series. This enormous creature swims through space, carrying the Discworld on its back, along with four giant elephants named Berilia, Great T'Phon, Tubul, and A'Tuin. The shell itself is described as being ancient and worn, reflecting the vastness of time and the weight of the worlds it carries. It symbolizes the interconnectedness of life and the universe, serving as a foundation for the diverse cultures and stories found within the Discworld series.