This article is incomplete. Please feel free to edit this article to add missing information and complete it. Reason: Full list of what items are available only if sewers are flooded, and layout
Castelia Sewers
ヒウン下水道 Hiun Sewers
"Wild Missingno. appeared!"
Map description:
Iron bars around the exits were dismantled, drawing curious Trainers.
The Castelia Sewers (Japanese: ヒウン下水道Hiun Sewers) is a location in south Unova. It is accessible via an entrance in Castelia City on 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.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Trainers
Even when partnered with Hugh, Trainers other than the Team Plasma Grunts are engaged only in Single Battles, without Hugh.
The Castelia Sewers made an appearance in A Venipede Stampede!. Burgh, Ash and his friends observed a Venipede stuck in one of the pipes and the Venipede got free with their help. Notably, this area appeared in the anime long before it was accessible in the games.