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.

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 - Ultramar
Planets: Armageddon - Bakka - Cadia - Catachan - Ganymede
Hydraphur - Krieg - Kronus - Sacris - Sanctuary 101
Scelus - Tanith - Tartarus - Vigilus - Vraks - Zayth
Daemon Worlds: Plague Planet - Sortiarius
Systems and Regions: Ghoul Stars - Halo Zone - Jericho Reach - Kaurava System
Solar System - Stygius Sector - T'au Septs - Taelus System
Types of Worlds: Agri-World - Craftworld - Daemon World - Death World - Eldar World
Forge World - Fortress World - Hive World - Tomb World