Rogueport Sewers

Rogueport Sewers is a huge chain of rooms under Rogueport that contain the Thousand Year Door, various warp pipes, and the Pit of 100 Trials. There is a hidden town here with a shop that sells rare items such as Ultra Shrooms and Gold Bars. Hidden in the sewers is a Teleportation Device that the X-Nauts used to transport to the sewers and back to X-Naut HQ. The main way into the sewers is a pipe in front of Professor Frankly's house. Another way in is West Rogueport's sewer grate. The pipe to Boggly Woods, Fahr Outpost, Twilight Town, and Petalburg are their, as is the other levels warp pipes. Throughout the game you'll be coming and going in here.

Other Notes
The Thousand Year Door is directly below the noose platform above Rogueport.

The Rogueport Sewers is to Paper Mario: The Thousand Year Door as Toad Town Tunnels is to Paper Mario.