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. There is also a badge seller named Dazzle, who trades them for Star Pieces. Merluvlee's house is also located here, where Mario can pay her to find the locations of Star Pieces or Shine Sprites, or what to do next. Lastly, there is Wonky's house who Mario can pay to give certain information about the game.

The main way of getting into the sewers is a pipe in front of Professor Frankly's house. Another way in is West Rogueport's sewer grate. Special Blue Pipes leading to Petalburg, The Great Tree, Keelhaul Key and Poshley Heights can be found in the sewers.

Regular portals to particular regions include a stone warp pipe to Petal Meadows, two wooden pipes: one to Boggly Woods and one to Twilight Town, although the one to Boggly Woods is monochrome like the rest of the woods and the one to Twilight Town is brown with exaggerated shadows, and a blue warp pipe is used to gain access to Fahr Outpost. The Teleporter is the fourth portal found in Rogueport Sewers and is used to reach the X-Naut Fortress and through that, the Moon after Mario is fired out of the cannon, as it is impossible to be fired again. After completing Chapter 7, this teleporter cannot be used until after completing the game.

When Mario reaches the bottom floor of the sewers for the first time, he has to learn a new paper ability to reach the door from the first Black Chest Demon inside a black chest. After Mario reaches the Thousand-Year Door, he has to stand on the pedestal and hold up the map. The map shows him the location of one of the Crystal Stars. This process is repeated after every Crystal Star Mario gathers.

A Squeek can be found here, looking for her lost partner while researching the depths of Rogueport Sewers.

In-game map description
"The ruins of an ancient town remain intact underground, and deep within them is the legendary Thousand-Year Door. Holding the Magical Map aloft before that door will lead to the Crystal Stars."

Other notes

 * Merlee's and Chet Rippo's houses in the background can be entered by special warp pipes.
 * As seen in one of the final cutscenes, the Thousand Year Door in the Rogueport Sewers is directly below the gallows in Rogueport Square. Logically, the fountain in the ruins of the old town is located directly between the two as well.
 * The Rogueport Sewers is to Paper Mario: The Thousand-Year Door as Toad Town Tunnels is to Paper Mario.
 * The spike room is nearly identical to a room in Tubba Blubba's Castle in the first Paper Mario and is cleared in a very similar fashion (requiring Vivian instead of Bow).

Inhabitants

 * Merluvlee
 * Dazzle
 * Eddy the Mask
 * A Squeek
 * A Goomba
 * The Toad manager of the Deepdown Depot
 * Herb T.
 * Wonky
 * A Bandit and a pink Bob-omb in Herb T.'s cola bar.
 * Chet Rippo
 * Merlee

Roaming characters

 * A Black Chest Demon who gives Mario the plane ability.
 * Blooper (tattle-able)
 * Punio (found during Chapter 2)
 * Pine T. Jr. (Pit of 100 Trials, during trouble)
 * Pine T. Sr. (Pit of 100 Trials, during trouble)
 * Charlieton (Pit of 100 Trials, every 10th floor except 100th)
 * The Mover (Pit of 100 Trials, random)

Enemies

 * Goomba
 * Hammer Bro (appears after obtaining the Sapphire Star)
 * Koopatrol (appears after obtaining the Sapphire Star)
 * Koopa Troopa (appears after obtaining the Sapphire Star; they appear during battles with Koopatrols, Hammer Bros., or Magikoopas))
 * Magikoopa (appears after obtaining the Sapphire Star)
 * Paragoomba
 * Paratroopa (appears after obtaining the Sapphire Star; they appear during battles with Koopatrols, Hammer Bros., or Magikoopas))
 * Spania
 * Spinia
 * Spunia (appears after obtaining the Sapphire Star)
 * Spiky Goomba
 * Blooper (Prologue Boss)

Items
Additional items are on sale at the Deepdown Depot as well as dropped by enemies.

Area Tattles

 * We're down below Rogueport. The current town was built on top of this ancient one. Pretty surprising to find all this down here, right? It kinda blows my mind... The ruins here are all crumbly and gnarly now, but it was once a great town, really.
 * We're down below Rogueport. Folks say these are the ruins of a 1,000-year-old town. Even though these buildings are crumbling, some seem safer than those above us. The people living here mostly can't show their faces above for one reason or another. Some have actually started successful businesses here, though! What a hardy bunch!
 * We're down below Rogueport. I think this is just your basic, garden-variety corridor. This must've been a nice walk when the whole town was aboveground... 
 * We're down below Rogueport. Y'know, this seems to be more than just a corridor... Nothing's ever what it seems down here, huh?
 * Who knew the area under Rogueport extended so far? This place is a total maze! You really feel the grandness of that ancient civilization when you're down here...
 * There's a black box in the middle of the room. Y'know, in case you missed it. And, um... Yeah, there's just not much else, really.
 * This is the site of the famed Thousand-Year Door. The air ripples with power... Behind that door sleeps the treasure that every rogue in Rogueport whispers about... I also read this was where the palace at the center of the ancient town was. Just a little fun fact, there...
 * Looks like some seawater's found its way in here, huh? Yeah, that's a health hazard. I bet some weird sea things live in there, too... so try not to fall in the water, OK? Oh! There's a pipe here that connects to Petal Meadows.
 * This is Merlee the charmer's house. With her charms, you get random bonuses in battle. It costs a few coins, but I hear it's totally worth it.
 * These spikes seem to rise and drop according to a set pattern... They must be here to keep out thieves, huh? Now THAT'S a security system! With good timing, if you ran while the spikes were down... you'd still be a shish kebab.
 * That's the entrance to the Pit of 100 Trials. No one knows how far down that pipe goes, but... it keeps me awake at night.
 * We're down below Rogueport. You could spend a lifetime studying secrets down here.
 * We're down below Rogueport. This corridor connects to the area above. Nice, huh? By the way, you know you can pass through iron bars in Paper Mode, right?
 * This is just another part of the underground corridor. I bet tons of people passed through here a thousand years ago. Coooooooool.
 * We're down below Rogueport. I never knew this room was connected like this... If you see a similar entrance in another area, you should totally check it out.
 * We're down below Rogueport. There's a pipe here that connects to Twilight Town!
 * This is the house of the level adjuster, Chet Rippo. Whenever you want to adjust your levels or your partners' ranks, just come here, OK? Seriously, though, how many customers do you think he gets, living down here?
 * Another room in here, huh? Wow. But what's the significance of it? An undiscovered chamber of purpose unknown... THIS is why I love archeology!
 * We're down below Rogueport. There's nobody here. Huh! I wonder where they went?