Bakka
The Segmentum Fortress of Segmentum Tempestus and thus headquarters of Battlefleet Tempestus. Bakka is a hellish young world, inhospitable even compared to forge worlds but rich in minerals. The Segmentum Fortress consists of a massive number of stations and shipyards orbiting the planet.
Bakka was the site of the Gareox Incident in M36, which led to the Imperial Navy's current distaste for carriers. Battlefleet Tempestus became divided into a traditionalist faction and a pro-carriers faction (which happened to be full of chaos cultists). A space battle ensued which went poorly for the carriers until they went openly traitor and fled.
N-not that it likes you... or anything.