From 1d4chan

The Segmentum Fortress of Segmentum Pacificus.

Hydraphur is an Earth-like world on the edge of its Segmentum. While often described as a forge world, the planet itself is not a holding of Mars, but rather the Administratum and the Ecclesiarchy. Hydraphur is encircled by an artificial ring of staryards which serve as the headquarters for Battlefleet Pacificus. With so many intersecting interests, Hydraphur is a jurisdictional nightmare and thus one of the few planets where the Adeptus Arbites engages in low level policing.

During the Plague of Unbelief Hydraphur yielded to the forces of the Apostate Cardinal. Many ships of the battlefleet were lost in Bucharis's ill conceived attack on Fenris.

The Planets, Systems, Regions and Sectors of the Galaxy
Imperial Homeworlds: Holy Terra - Sacred Mars
Primarch Homeworlds: Baal - Barbarus - Caliban - Chemos - Chogoris - Colchis
Cthonia - Deliverance - Fenris - Inwit - Medusa - Nostramo
Nocturne - Nuceria - Olympia - Prospero - Macragge
Notable Imperial Worlds: Armageddon - Bakka - Cadia - Catachan - Cretacia - Ganymede
Hydraphur - Necromunda - Krieg - Kronus - Phyrr - Pythos - Sacris
Sanctuary 101 - Scelus - Scintilla - Tanith - Tartarus - Titan - Vigilus
Vraks - Zayth - 108/Beta-Kalapus-9.2
Daemon Worlds: Bathamor - Black Marble - Fleshworld - Glass Moon
Medrengard - Plague Planet - Sortiarius - Sicarus
Systems and Regions: Ghoul Stars - Halo Zone - Jericho Reach
Kaurava System - Solar System - Stygius Sector
T'au Septs - Taelus System - Ultramar
Types of Worlds: Agri-World - Craftworld - Daemon World - Death World - Eldar World
Forge World - Fortress World - Hive World - Civilised World - Tomb World