The Castelia Sewers (Japanese: ヒウン下水道Hiun Sewers) is a location in central south Unova. It is accessible via an entrance in Castelia City at the end of Thumb Pier. They cannot be accessed until the player talks to Clyde in front the Castelia Gym. When exploring for the first time, Hugh will join the player as a Multi Battle partner. Here the player meets Colress for the first time. Relic Passage is accessible from here. There is also an exit to Castelia City's park with wild Pokémon, and when the sewers are flooded, to Castelia City's back alley. During autumn and winter, the sewers are dry, blocking some areas and removing the requirement of Surf to access others.
A female Scientist in a room near Doctor Heath and a male Scientist in a room near Janitor Brady are attempting to turn poison into medicine, and will give the player items created in their experiments. They can give out any one of these items on a daily basis.
The Castelia Sewers appeared in A Venipede Stampede!. Burgh, Ash, and his friends observed a Venipede stuck in one of the pipes and it was freed with their help. Notably, this area appeared in the anime long before it was even accessible in the games.
In Angry Boy, Roxie discovered a man who had been attacked by Team Plasma, who stole his Karrablast and ran off into the sewers. Blake, Whitley, and Hugh each headed into the sewers with Roxie to find the culprits. During an encounter with a group of Team Plasma Grunts, the group got scattered, with Hugh then being found by Looker's Croagunk. Using a Xtransceiver on Croagunk's hat, Looker tried to question Hugh about what he knew about Team Plasma, but Hugh refused to tell him anything. Meanwhile, Whitley managed to rescue the Pokémon stolen by the Team Plasma Grunts with the help of Foongy, while Blake effortlessly captured Bronius and Ryoku before heading out to investigate the source of the cold air coming out of the Relic Passage.