Goop

"As you are no doubt aware, someone has been senselessly defacing fair Isle Delfino using some paint-like substance. The accused is charged with polluting our beautiful home and yes, endangering our very way of life. Indeed, how can one not be aware of what is going on? Though it is daytime in Delfino Plaza, our poor residents tremble beneath a veil of darkness. Expert Shine scholars have determined that this darkness has arisen because all of our guardians, the Shine Sprites, have vanished from their gathering spot at the Shine Gate. The reason? It's quite obvious. This horrible graffiti is to blame!"

- Pianta prosecutor

Goop, also known as graffiti (though this more properly refers to a variety of goop that is painted on walls), gunk, or sludge, is a hazardous substance that first appears in Super Mario Sunshine. It is a thick, slimy, paint-like residue that can slick Mario's footing and deal damage to him, as well as causing other characters and structures to sink into it. It can be cleaned up and removed using water, most notably sprayed by F.L.U.D.D.

Super Mario Sunshine
Goop is a major hazard in Super Mario Sunshine. It is spread across Isle Delfino by Bowser Jr. in the guise of Shadow Mario using his Magic Paintbrush. It is first encountered in the opening cutscene taking place on the Delfino Airstrip, where a pink, orange, yellow, and white color variant has been painted over the main airstrip, forcing the Toad Express to curve and skid to a halt as it lands. After dismounting the plane, the yellow Toad in Princess Peach's entourage notes that the goop is moving on its own. Mario subsequently obtains FLUDD on the airstrip and uses it to clean up the goop and defeat a Gatekeeper, a type of large, goop-based Piranha Plant dwelling in the middle of the goop, restoring the airstrip to its normal state. Mario is arrested by the Isle Delfino Police shortly afterward and accused by the Pianta prosecutor of painting the goop all over Isle Delfino. The Pianta judge then sentences him to clean up all of the island's goop and graffiti.

Ordinary goop is a tar-like substance that slickens the ground and causes Mario to lose traction. If Mario stands on goop for too long, he sustains damage every few seconds. It also dirties and sticks to his clothes, which can be cleaned by spin-jumping or swimming in clean water. Goop is found in several different colors depending on the area in which it is encountered. According to a certain Pianta in Delfino Plaza who is trapped in it during Mario's first visit, it tastes "sweet," and he likens it to "candy."

Except on the Delfino Airstrip, goop regularly spawns Swoopin' Stus, small enemies that seek out and attack Mario when he is nearby, spreading more goop in the process.

Goop appears in three different colors depending on the area and scenario it is encountered in:
 * Pink, yellow, orange, and white goop appears on the Delfino Airstrip during Delfino Airstrip Dilemma as well as in Episode 1 of Noki Bay (Uncork the Waterfall).
 * Brown-and-white goop appears in Delfino Plaza's Polluted Piranha Battle and Shadow Mario Chase events, as well as Episodes 1, 2, 3, and 5 of Bianco Hills. Petey Piranha launches this color of goop from his mouth, and ordinary Piranha Plants require it in order to appear and attack Mario, as well as being covered in it. Piranhabons are similarly covered in it, and Coo Coos generate it in Bianco Hills.
 * Black goop appears in Delfino Plaza's Boathouse Blues and Cleaning Lighthouse events, as well as Episodes 1–5 of Ricco Harbor and Episode 2 of Noki Bay (The Boss of Tricky Ruins). It is spat out by Gooper Blooper and ordinary Bloopers as a projectile, resembling ink in this way.

Generators are undulating, smoking masses of goop that house Gatekeepers, which can be released by repeatedly spraying the Generator with FLUDD's Squirt Nozzle. They appear on the Delfino Airstrip and in Delfino Plaza, Bianco Hills, and Ricco Harbor. Their colors match those of the surrounding goop. As their name suggests, the Generators that appear over Delfino Plaza’s boathouse and lighthouse generate goop that cannot be permanently cleaned until the Generator is destroyed by defeating its Gatekeeper; while it remains, the goop regenerates a moment after being cleaned. The Generators on the Delfino Airstrip, over the Grand Pianta Statue in Delfino Plaza, and in Bianco Hills do not have this property.

During Episode 1 of Noki Bay (Uncork the Waterfall), blotches of pink, yellow, orange, and white goop appear on the walls of a tall cliff face near the starting point of the level. These goop blotches are shaped like various things relating to the sea, including a squid, a sailboat, a dolphin, a whale, an octopus, and a bird. This goop can be cleaned off by Mario, but it cannot hurt him. As Mario cleans the blotches off, rock formations of varying sizes appear out of the cliff face, and he can then use them to climb upward. Cleaning off all of this goop helps Mario reach the cork plugging the waterfall, where he has to defeat a Monty Mole in order to finish the episode.

There is also black goop on the wall near the crates where Gooper Blooper is fought for the first time in Ricco Harbor. It cannot be washed off, but it does not hurt Mario. The goop leaks out of the crates Gooper Blooper is in and runs down the wall into the water, turning it into slimy water.

Along with electric goop and burning goop, regular goop can trap many friendly characters, including Piantas, Nokis, and Toads (including Toadsworth) if they are standing on it, causing them to sink into it. They are freed after the goop around them is cleaned up and can subsequently be sprayed clean, causing them to celebrate and thank Mario if spoken to. Certain Piantas and Nokis may reward Mario with a coin, Blue Coin, or 1-Up Mushroom when spoken to after being rescued this way. Piantas that are dirty with goop but not trapped in it also appear in Bianco Hills and Pianta Village (specifically, the Pianta mayor is covered in burning goop in the latter case), and they reward Mario similarly if cleaned and spoken to, with a Shine Sprite in the case of the Pianta mayor.

Terrain features and structures, including trees; rock formations in Noki Bay; a section of the Delfino Airstrip; Delfino Plaza's Grand Pianta Statue, boathouse, and lighthouse; the hill leading up to the Big Windmill in Bianco Hills; and Sirena Beach's Hotel Delfino, can sink into regular goop and electric goop as well. Trees and Noki Bay's rock formations sunk this way can be released by cleaning up the goop around them, and certain trees may be revealed this way by spraying patches of ground in Gelato Beach as well, which does not contain any regular goop or electric goop. Other structures sunk this way can typically be released by defeating a Gatekeeper residing on top of the sunken structure, though in Hotel Delfino's case, the Manta must be defeated instead. In all cases, objects trapped in goop display a faint bubbling animation when released, with an earthquake occurring as they emerge.

Super Mario 3D World + Bowser's Fury
While regular goop does not appear in Super Mario 3D World + Bowser's Fury, graffiti and a new variant called splotchy goop do.

Mario Golf: Toadstool Tour
In Mario Golf: Toadstool Tour, Petey Piranha spits up brown-and-white goop when he gets a double bogey or worse.

Mario Golf: Advance Tour
In Mario Golf: Advance Tour, goop appears on the Mushroom Course along with many Proto Piranhas. Landing the ball in goop counts as out of bounds.

Mario Power Tennis
In Mario Power Tennis, brown-and-white goop appears as an obstacle on the Delfino Plaza Court. It can cause players to slip and slide around. The player must also clear away goop in a tic-tac-toe-like minigame called Tic-Tac-Glow. Additionally, Petey Piranha utilizes brown-and-white goop in his Power Shots, Sludge Racket and Piranha Swingback.

Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
Goop appears in Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U as part of Bowser Jr. and the Koopalings' Final Smash, Shadow Mario Paint. This move involves the user transforming into Shadow Mario and using the Magic Paintbrush to paint yellow-and-orange goop over the screen in an X shape, which can deal damage to opponents on contact and eventually explodes. The goop painted visually resembles Super Mario Sunshine's regular goop and burning goop but in the shape of X Graffiti.

Super Smash Bros. Ultimate
Goop reappears in Super Smash Bros. Ultimate, once again painted across the screen by Bowser Jr. and the Koopalings' Shadow Mario Paint. However, it now remains onscreen for a shorter period of time before exploding.

Mario & Luigi: Paper Jam
In Mario & Luigi: Paper Jam, during the Airplane Guard sequence of the battle with Petey Piranha, he attacks Mario and Luigi by spitting balls of goop as he does in Super Mario Sunshine.

WarioWare Gold
In WarioWare Gold, Mario uses F.L.U.D.D. to clean brown-and-white goop off a building in the Super Mario Sunshine microgame.

Dr. Mario World
In Dr. Mario World, Dr. Petey Piranha uses goop to destroy viruses during his special skill animation.

Variants
Numerous varieties of goop have appeared, largely in Super Mario Sunshine, consisting of the following:
 * Burning goop, a fiery orange variant of ordinary goop that appears in Pianta Village
 * Electric goop, an electrically charged green-and-yellow variant of ordinary goop that appears in Sirena Beach
 * Glorpedoes, rolling ball projectiles formed from goop that appear in Bianco Hills and Noki Bay
 * Graffiti, a dried form of goop used by Bowser Jr. to tag walls in Super Mario Sunshine and Super Mario 3D World + Bowser's Fury
 * Hot water, slimy green goop that appears in the final boss battle against Bowser and Bowser Jr. in Corona Mountain
 * Orange Juice Generators, pulsating masses of orange goop that can be destroyed only by a Yoshi's juice-spraying ability
 * Rainbow Ms, an amorphous, multicolored form of goop that can be painted on walls. Mario and Shadow Mario can use it to travel to other areas of Isle Delfino. Similar goop also appears within the pipes to Sirena Beach and Pianta Village.
 * Slimy water, damaging, goop-polluted water that appears in Bianco Hills, Ricco Harbor, and Noki Bay
 * Splotchy goop, black goop that corrupts Bowser and pollutes the waters of Lake Lapcat in Super Mario 3D World + Bowser's Fury

In addition to these variants, most enemies and bosses in Super Mario Sunshine, as well as the game's Yoshis, appear to be at least partially composed of goop, as they dissolve into goop bubbles when attacked or defeated. This is most noticeable with enemies such as Strollin' Stus, which emerge out of a Generator-like mound of goop in Episode 1 of Bianco Hills (Road to the Big Windmill), but also occurs for enemies and bosses such as Cataquacks and King Boo. The Super Mario Bros. Encyclopedia confirmed that the enemies and Yoshis in Super Mario Sunshine are created from Bowser Jr.'s graffiti.