Ganymede

Ganymede is Jupiter's largest moon and the old hub of the Jovian yards, later rebuilt as Hub-Fortress Aquila Adamant for Guilliman's Indomitus fleets. Abandoned Mechanicum labs still riddle the crust. Daemons took the moon in the Noctis Aeterna; Grey Knights burned them out while tech-priests were already sinking data-engines toward the core.