Rogueport Sewers is a location in Paper Mario: The Thousand-Year Door.
One thousand years before the events of the game, it was the location of a civilization. Long after its decline, people built Rogueport over top of it. Two notable locations found within these halls are the Thousand-Year Door and the Pit of 100 Trials. There is also a small town down there and a shop, named Deep Down Depot, that sells Gold Bars. This shows that people have actually started successful businesses down here. Mario, with the help of Goombella and Professor Frankly, go through here at the very beginning of his journey. Throughout his time there, he finds a Black Chest that houses some sort of Demon. This Demon then "curses" Mario to be doomed to turn into an paper airplane when you press "Y" on the airplane spaces.