From Bulbapedia, the community-driven Pokémon encyclopedia.
Poison Swamp (Japanese: どくのぬま Poison Swamp) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It is available for 8,500 Poké from Wigglytuff in Pokémon Square. It is in the Marsh area, and has the capacity of six Pokémon. All Pokémon here are of the Poison-type.
It is a strange purple swamp, with odd blue mushrooms supporting platforms and stairs.
A horrid swamp that spews toxic gases. It is literally a utopia for Poison-type Pokémon.