From the Super Mario Wiki, the Mario encyclopedia
Graffiti Underground is a location in Paper Mario: The Origami King, an underground sewer area with graffiti and a valve controlling the drain. It is used to reach Peach's Castle from Toad Town, and is accessed via the Manhole Hook. Folded Soldier Swoops and Scaredy Rats are introduced here. There are also a few areas throughout the sewer where Mario must hit a red button to light up the area.
Graffiti Underground is a small, simple area. Once Mario has acquired the Manhole Hook from a Shy Guy in Toad Town, he can pop open the sewer lid and travel through the sewer to reach Peach's Castle as an alternative to crossing the regular bridge, which is now broken. Right after Mario descends the ladder, Olivia asks him to hit a nearby button to light up the place, revealing a drawing of Mario and Princess Peach on the wall. To the left of the ladder is a Not-Bottomless Hole in the wall. To the right is a Toad huddled in the corner. When he notices Mario and Olivia, he gets up and introduces himself as curator of the Musée Champignon, which can now be accessed.
At the start of the next area is another button Mario must hit to light it up, revealing origami Swoops on the ceiling, which drop down unexpectedly on Mario. After a Not-Bottomless Hole, there is a set of stairs leading down to a Heart and the entrance to the next area, where there is another button and a ? Block containing a Super Mushroom, guarded by origami Scaredy Rats. As Mario starts to advance through this section, he is stopped by Scaredy Rats coming out of the wall and Swoops coming in from the left, forcing him to battle. He can then move through to the next area.
The next area has yet another button Mario can hit to light it up, revealing a swarm of origami Swoops and a small pool of water in front of the pipe leading up to Peach's Castle. To reach it, Mario must climb down a ladder and use the 1,000-Fold Arms to pull the valve, effectively draining the pond. Mario can then access the pipe as well as the Coins and treasure chest in the drained pond, where he can find a collectible treasure. To the right of all this is a separate section with a crate floating in the water, guarded by two origami Scaredy Rats. Mario must defeat them and break open the crate to reveal a Toad, who tries to thank him before floating down a tunnel.
If the player returns to the Graffiti Underground after clearing more of the game, more and more graffiti is added; the full set of pictures depicts Bowser attacking and defeating Mario, Luigi, and the Toads, only for Princess Peach to defeat him with her umbrella.
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