Imagine you're like the fabled King Midas, and you have the power to convert matter with a single touch. Except that instead of gold, everything you touch turns into shit. And everything it touches turns to shit. Before you know it, the whole world is shit, and it's all your fault.
Grey Goo is like a Horde of Alien Locusts, only replace Alien Locusts with Nanotechnology — and instead of grass, they "eat" anything. Or, if the protagonists are lucky, just anything mineral or electronic.
They're worse than alien locusts. Grey Goo destroys resources by turning them into more grey goo — more nanomachines. It's The Virus for nonliving things — though it may be able to take down living things as well, and likely will turn them into nonliving things if they're in the wrong place. In theory, you can end up with a planetary body made of nothing but grey goo. Physical laws regarding energy, thermodynamics and the like stand in the way, but even a partial success in this case is likely to suck for everyone involved.
The color is conventional. Other colors of goo that work the same way are just as bad. Red Goo or Black Goo are intentionally released nanoweapons that do the same thing, and may or may not have an off switch, which may or may not work in any case. Green Goo doesn't disassemble everything, but just kills living things, or renders them infertile. Blue Goo is a defensive "antibody" developed to fight back against another scenario; if there's a nanobattle inside your body, the waste heat might cook you to death anyway. Golden Goo is a specific grey goo variant, where nanomachines designed to pull gold from seawater disrupt the ocean ecosystem, or less probably, dry up the oceans. Pink goo are people. No, not like that but it is the cause of it.
For some reason, most of the time these specifically were created to clean up oil spills.
Can cause The End of the World as We Know It — specifically, Type 3b or 4 on Apocalypse How.
Compare Blob Monster.
Grey goo has completely devoured Mercury by the time of Gunnm (AKA Battle Angel Alita).
The WORMS in Sky Girls are related to this. Sort of.
∀ Gundam features Black Goo in the form of the Moonlight Butterfly (Gekkōchō), which is named because it manifests as giant shimmering energy contrails that emerge from the titular Humongous Mecha's back like wings. The nanomachines only target technology, but do so on an immense scale; the last time the Moonlight Butterfly was used, it sent humanity into a Dark Age that they're still recovering from (when the series begins, technology is roughly on par with the early 1900s).
In creator Yoshiyuki Tomino's original novel, the full powered version of the eponymous Gundam can affect the area from Earth to Jupiter with the Moonlight Butterfly - that's over 600 million miles.
The DG Cells in G Gundam are a combination of this and The Virus, though strangely enough they were designed to be beneficial, as their original purpose was breaking down dead or decaying matter and using it to foster new life. The Devil Gundam instead uses them to reanimate dead humans under its control, or infect live humans and Mind Control them.
Adam Warren's adaptation of the Dirty Pair revealed that the Earth had been destroyed decades earlier in a massive Grey Goo outbreak, the "Nanoclysm", which led to nanotechnology being regulated and virtually outlawed. The villain of the miniseries planned to use a cache of nanotech to take over Heroes R Us's Central Computer, and from there, the known universe. Unfortunately, the Central Computer revealed that it was partially based on something the Nanoclysm left humanity as an apology...
His later Empowered had a Grey Goo eruption that created Sexbots, for reasons too complicated and silly to explain but which involved someone trying to use alien nanobots with an untranslated interface for perverse purposes.
The Filth features creatures like this, but portrays them in a very sympathetic light during the stages of their evolution. The more they spread, the more the world is seen from their perspective.
The Modular Man from Tom Strong is a hi-scale example of this. Each individual module is about the size of your head. Once he gets to Venus, though, he multiplies until he has something closer to the proper Grey Goo appearance.
Transmetropolitan mentions it as a possible weapon if the commonly-used "makers" are reprogrammed. The standard MO for handling a "grey goo" scenario is to release "blue goo" (disassembler nanites that degrade matter — all matter — to the point where Grey G can't make more nanites out of it, and then destroy the Grey Goo nanites) to contain the grey goo and restrict its damage to a small area. One brief mention is made of someone who lost his legs because he decided to (and succeeded) shut off the grey goo instead of releasing the blue goo.
One of the early comic issues of MAD had the planet Mars consumed by the Gookum, a jelly-like pink substance which eats anything organic, and, worse, It Can Think. It stays dormant for 500 years at a time, and the joke/Mandatory Pulp Sci-Fi Twist is that cherry Jello parfait is completely indistinguishable from dormant Gookum. One day... one day the Gookum will quiver, will stir...
One issue of Fantastic Four claimed that the reason Reed has never tried to market the unstable molecules the team's suits are made out of is that if a careless person gets ahold of the stuff and pokes it the wrong way with an electron microscope, it starts destabilizing all surrounding matter into an ever-expanding blob of goo.
It also had a limited life span which meant that if it's food source was too far away it couldn't make it, depending on how much of a start it got. Eating the Eiffel Tower would give it enough of a start to devour all of Paris, but it would likely die out before it reached another city. Devour a single ship high in the stratosphere and it dies out long before it reaches anything else.
It's unlikely that the nanomites were self-replicating at all. A self-replicating nanomachine is by necessity, much more complex that a simple molecular disassembler. And if you're foresighted enough to build in a kill-switch, the very last thing you'd want to risk is the possibility of generational copying errors disabling that function.
The Blob (in the original 1958 movie) could be either this or The Virus. (No one's really ever gotten close enough to examine it without being eaten. All that's known about it is that it's of alien origin. Oh, and it prefers to devour organic life as opposed to inorganic matter.) The 1988 remake of the movie had the Blob be the spawn of a secret government germ warfare project. (And there it acted less like mindless spreading Grey Goo and more like a malicious, semi-intelligent monster.)
Century Rain by Alastair Reynolds has Earth effectively uninhabitable from the effects of rogue swarms of nanobots. The nanobots were originally created to affect weather patterns, but they were corrupted. New bots were created to fight those, which went rogue, and so on and so forth. The survivors from the disaster, who now reside in orbital habitats, weaponized the Grey Goo and use it as a weapon of mass destruction.
Something similar happens in his Revelations Space universe, where The Greenfly terraformers go rogue and will inevitably consume the entire universe.
One of the best examples is Wil McCarthy's Bloom, a novel set in a future where the last fragments of humanity lives in habitats in the asteroid belt and in jovian orbit, after the entire inner solar system was devoured in a Grey Goo incident. Earth, Venus, and Mars are now large fuzzy balls of nanotech, nanotech solar sails drift aimlessly throughout the system, and occasionally errant strands drift out on the solar winds and try to devour anything they touch in the outer solar system.
It's okay, though, because the billions of people devoured in the incident are merely compressed, not gone, and their brain engrams still survive in the bloom, immortal and living in virtual paradises of their own construction; imagine how powerful a computer would be if it was made from an entire solar system?
Another über-example would be Charles Stross' Accelerando, where pretty much the same events as in Bloom happened deliberately, and for the betterment of mankind.
The novel How to Mutate and Take Over the World ends with nanites from a desert factory transforming the world into key lime pie. And this gets spoiled about a third of the way in, in a fictional review of the book.
In one of the Thursday Next novels, Thursday's time-traveling father tells her of a future wherein the world was overtaken by such a scenario; the world is consumed by pink slime. It turns out to be strawberry pudding.
Death from the Skies by Phil Plait presents a berserker Von Neumann probe, which is essentially grey goo on a cosmic scale.
Neatly inverted in Charles Stross' Saturn's Children—robots think of organic life as "pink goo," reproducing without limit.
In Scott Westerfeld's book Specials, the main characters break out of a weapons storage facility using nanotechnology-based silver goo (much more flashy and dramatic than plain old gray goo, to paraphrase the author).
In The Forge of God, this is done deliberately and systematically by a belligerent alien race, to humanity as well as at least one other race. The sequel, Anvil of Stars, is the story of a handful of the survivors of Earth — specifically, the children — seeking out the race that destroyed Earth, to enact the Law.
In Greg's Blood Music, the Green Goo is not nanotechnology, but biotechnology, but operates in basically the same fashion. The "noocytes" consume all the biomass in North America, converting it to more noocytes. It turns out that all the living creatures who are assimilated are also recorded and "alive" in a new kind of reality, similar to "Bloom" above.
The Melding Plague also ended humanity's golden age in Alastair Reynolds' Revelation Space series. Everybody was enjoying the benefits of a highly advanced nanoscience, until humanity encountered a plague that could subvert all but the hardiest nanotechnology. Such subversion has disastrous consequences for the device in question, anything to which it was attached, and potentially anything nearby. This is partly used to justify the somewhat Schizo Tech nature of the setting.
Played with in The Dervish House by Ian McDonald. The Grey Goo scenario is a common fear in the near-future setting due to a famous scientist's doomsday predictions of a nanotech catastrophe, even though it's generally accepted that he was just fearmongering and was wrong. A character later comments that the Grey Goo scenario has already happened; it's called organic life. It turns out that self-replicating nanotechnology is key to a massive terrorist attack, but not in the traditional sense.
In the Star Wars RPG supplement The Unknown Regions, a Grey Goo named Mnggal-Mnggal is mentioned. It is sentient, and able to infect any form of matter, as well as take control of creatures as 'zombies'. Its origins are unknown, and it's likely biological. Note: If there was a trope for biological Grey Goo Mnggal-Mnggal would fit there.
In Jack McDevitt's Chindi, an ancient alien satellite that has already mysteriously destroyed one ship is being much more carefully investigated by a second when the ship begins to come apart. It turns out that the satellite uses grey goo to repair/replicate itself, and the ship is being eaten. Several people die, and one, the artist with a big crush on Hutch, is rescued just in the nick of time.
In John C. Wright's Count to the Eschaton, the Savants used Golden Goo. It destroyed the cities, where it was released first — killing most people there, with only a few stunned to be transferred with their minds intact — and only by savage, fiery attack by the Giants was the earth saved from their "Golden Age."
In Walter Jon Williams's Aristoi, a form of grey goo (called "mataglap nano") destroyed Earth That Was. As a result, only the eponymous Aristoi are legally allowed to use nanotech freely and they are fanatically paranoid about the possibility that any given form of nanotech could somehow mutate into mataglap.
In the short story Mar Pacifico by Greg Mellor, nanotech designed to fight the effects of global warming (by letting the oceans absorb more carbon) sweeps over the Earth absorbing most forms of life. When the protagonist is absorbed, she discovers that humanity still exists as a linked Hive Mind inside the nanotech.
A failed pilot for a TV series called Doorways by George R. R. Martin featured a parallel dimension where, yep, nanomachines used to eat up oil spillages went and ate all the oil. This scenario was the result of Executive Meddling — G.R.R.M's original script (and the one found in his "Dreamsongs" retrospective compilation) featured the parallel Earth as a Winter World, but this was apparently too bleak for a first episode.
The Replicators of Stargate SG-1 are this trope scaled up to lego size. When they eventually evolved to silver goo, their diet changed from "any kind of metal" to "neutronium only", thus keeping the new model a rarity.
A minor example of this was the nanite colony Wesley Crusher was running in one episode of Star Trek: The Next Generation. Fortunately, this goo turned sentient and was willing to be moved to a better food source before it disabled the ship.
Gray Goo is discussed in GURPS: Ultra-Tech in a section on Von Neumann machines and points out the waste heat of the goo eating a planet is likely a more pressing threat than being eaten by it. On the up side they require extremely high level technology and are expensive to make, on the down side some versions might be able to fly or travel through space.
Centauri Knights, a far future/Humongous Mecha sourcebook for Big Eyes, Small Mouth, takes place on a dead alien planet colonized by humans. According to the Game Master information in the back, the reason the planet was vacant was because an ancient war resulted in a Grey Goo superweapon accidentally being unleashed upon the planet (the game even explicitly uses the term "Grey Goo" to explain the phenomenon).
Dungeons & Dragons has this in the form of green slime, which converts anything it touches into more green slime. (Except stone, which is immune to it, and wood is more resistant than most. It dissolves metal very fast, however.) Fortunately, it's immobile, so you can avoid it if you just stay away from it.
Tasty Planet is a game based on Grey Goo where a cleaning agent gets bigger and bigger as you guide it through the levels starting on a Petri Dish until it eats the planet, then the solar system, galaxy, universe, space and time! Then it explodes and everything starts over.
The True Final Boss of Super Robot Wars Original Generation, Septuagint, can be seen as this. It may be more of purple crystals, but its directive is clear: exterminate the whole world by consuming anything to come to its path, and it can even make crystal-based duplicates of things it has consumed.
Mass Effect has Zaherux, a planet covered in silicon "seas" that has unmanned probes sent to it crash, a popular extranet meme goes that the seas that cover it are actually "disassembler" nanites.
Biological version in Outpost 2: Divided Destiny. A terraforming microbe runs wildly out of control, breaking down organic matter, among other things, and forcing both colonies on the planet to try to evacuate-one of the rare versions in which the Goo truly is unstoppable, though one colony can delay it briefly, and the omnipresent threat during the campaign.
In Hostile Waters, the "alien" antagonists have a grey goo Disassembler cannon. It fires at a city and reduces it to mush. It's up to you to blow up the cooling radiators before it fires its third salvo and destroys Central, the world capital. Once you do that, the next shot blows it to hell and spreads disassemblers throughout their base.
And in the first Deus Ex game, the Gray Death is revealed to be an artificial plague.
In Sid Meier's Alpha Centauri, one of the Special Projects shows the video of several containers placed on the site of a battle, littered with debris and dead bodies. The containers open, releasing nanites that look like glowing goo. They proceed to consume everything in sight, including the dead, and use the materials to create a brand-new Hover Tank.
Tiberium in the Command & Conquer universe can be seen a slow-acting example of it. It needs around half a century to engulf a significant portion of the Earth.
Grey goo missiles called the "Nano Virus" are a high-level planetary siege weapon in Sword of the Stars. It is amusingly classified as a bio-weapon, meaning you have to go through several tiers worth of gene modification to access it. The Nano Virus is harmless to organics, but the planet's industrial output will be heavily damaged and it will wipe out an AI rebellion as if they were living creatures hit by a regular bio weapon. It is the only bio weapon that will affect the Zuul, since their machinery is made of the same metal as everyone else's.
Parasite Eve features massive amounts of Pink Goo. The Big Bad is actually a Patient Zero infected with intelligent pink goo by accident. Every enemy in the game was created by the pink goo. If you go on to the Chrysler Building in the EX game, the building itself is infected with the pink goo (which is the in-world reasons that mode has random maps) and pink goo is literally everywhere.
A grey goo like attack is possible in Supreme Commander 2. The Cybran Nation can upgrade their engineers to have weapons, by doing this and building nothing but engineers you will get an ever growing blob of engineers that will automatically shoot every enemy in sight and use the remains to build more engineers. Full instructions here.
In Smoke's ending in Mortal Kombat: Armageddon, the power of Blaze causes his nanobots to go into overdrive, consuming all of Edenia and replacing it with a sentient mass of grey goo that calls itself Smoke.
The Vasari from Sins of a Solar Empire, who specialize in Nanotechnology, have ships that are able to throw blobs of destructive nanobots at enemy ships.
Provides the backstory and setting for Hawken. In the endless Corporate Warfare on the planet Illal, one of the three dominant Mega Corps collapsed, triggering a self-replicating nano virus that converts the evironment into a labrynth of patchwork metal—known as the "Giga Structure" and "Hawken Virus." At the time of the game's launch, the virus covered a third of the planet.
This page of xkcd jokes about how IPv6 is perfect in that the nanobots will only be able to devour about half the planet before they run out of addresses.
Hostile nanoswarms are so common in Schlock Mercenary that there's standard-issue tactics and equipment to stop them; people worried about nanobot infection drink nanotech-fighting chemicals to control them, "nanofilm" is routinely employed to control rogue nanobot swarms, and worst comes to worst, the nanobots will be isolated by ubiquitous AI with gravity-control technology.
Employer of the month: We've all been drinking Nanneze like it was Ovalquik.
In the late 20th century of the Chaos Timeline, nukes are scrapped because nanobots made them obsolete. The so-called Braunschleim scenario is the casual armageddon scenario everybody fears. On the eve of World War Three, this fear urges a bunch of Playful Hackers to seize control over the military and the rest of the world, in order to prevent the danger of nano annihilation.
One of Ben 10's alien forms is Upgrade the Galvanic Mechomorph, a piece of Grey Goo from a planet of sentient Grey Goo that has the ability to separate itself into smaller, independent entities that get less intelligent the more they split. They were created by the series' resident Sufficiently Advanced Aliens, the Galvans, but developed a mind of their own.
Inverted in another episode, in which some nanobots released on a deserted planet first eat some waste oil, then start trying to eat clothes, but soon get bored of that and create an entire ecosystem made of nanobots instead of cells. Where the necessary mass came from is left as an exercise for the reader.
Matrix from Gargoyles, a semi-sentient mass of nonobots created by Fox's mother (who mentions the Grey Goo scenario by name) in Australia, tries to bring "order" to the continent by eating and assimilating it. Dingo made a Heel-Face Turn and convinced Matrix to fight for "Law and Order" (Long story involving the Aborigine Dreamtime, roll with it), so it fused itself with his Power Armor. Later, he did provide one of the funniest lines in the comic series on taking over the world.
An episode of the 1980's animated The Incredible Hulk involved a scientist bioengineering a Blob Monster that could eat literally anything except the special glass of its container, and would get bigger the more it ate. Of course the glass breaks and it starts eating Gamma Base. Fortunately, it turns out that the one thing it's allergic to is gamma radiation, which the Hulk constantly emits.
The Justice League Unlimited episode "Dark Heart" dealt with alien nanomachines that were in the process of taking over Earth this way. Their species appears to be made artificially for a war from fifty-thousand years ago, and is meant to consume every planet they're sent to, spread to some other planets, and repeat until they're all dead.
The G1 Transformers episode Kremzeek! involved a highly specialized electricity eating spark creature, splitting into new copies of itself as it ate more and more current.
The "annoying replicating electrical prankster" is a kids' show plot that really gets around. In the old Teenage Mutant Ninja Turtles cartoon, it's called The Big Zipp Attack, in The Mighty Ducks it's "Zap Attack." Ben 10 has the imagination to not name it after the creature: instead of something like "The Megawhatts Attack," the critters are first encountered in Tourist Trap. (We find they're actually an alien race called Nosedeenians.) It's not as obligatory as the Fantastic Voyage Plot, but watch enough cartoons and you'll know it by heart.
One episode of The Powerpuff Girls features Grey Goo nanobots dropping from a raincloud and destroying Townsville, though the bots thankfully ignored living creatures.
In the Code Lyoko episode "Marabounta", Jeremie tried to create a virus that would destroy XANA's monsters, using Franz's Hopper's journal and basing it off African army ants. At first the huge blob-like monster seemed to work, attacking and devouring XANA's minions. Then something went wrong, and it tried to attack Aelita. Jeremie realized too late that her connection to XANA was causing it to mistake her for one of the monsters, but by then it was out of control and growing larger by the minute, threatening to consume all of Lyoko. (Leading to one of the most chilling scenes of the series to date, Yumi trying to protect Aelita, only to be devirtualized when she was Eaten Alive by the thing.) Fortunately, XANA, who at that point needed Aelita alive to complete his plans, instructed his monsters to help the heroes, and due to the brief alliance, they bought enough time for Jeremie to program and anti-virus that eradicated it.
An earlier episode "Amnesia", had XANA infecting nanobots from a class as an infestation to cause massive amounts of memory loss.
Life itself has spread to every conceivable corner of the Earth (that we have been able to check so far) in the last 4.5 billion years and readily converts non-living matter into living matter through it's inexorable reproduction. It is also really really difficult to truly sterilize anything. This is sometimes referred to as the 'Green-goo' scenario.
A strange example from Real Life which has elements of the Grey Goo scenario: In the 1950's scientists managed to isolate an immortal cancer cell line (that is, undying cells that could be grown outside of the human body) which they named HeLa, after Henrietta Lacks, an unfortunate woman who had died from the cancer. Scientists at the time were trying to isolate and grow normal human cells outside of the body but were having no luck. Within a short period of time, however, they noticed many of the normal cells suddenly being transformed into cancer cells. This puzzled scientists until they realized that the new cells were actually HeLa cells that had contaminated the normal cultures.
HeLa cells were so durable that they were difficult to get rid of via sterilization and the tiniest bit could convert whatever normal tissue they came into contact with. HeLa cells soon spread throughout the world and even today scientists have problems dealing with HeLa contamination of their cell cultures.
Fortunately for humanity, the HeLa cells quickly die out or result in only weak tumors when injected into a living human body (yes, there were people crazy enough to try this out). Even more fortunately for humanity, the HeLa cells ability to grow outside the human body led to it being a useful tool in the development of the Polio vaccine, in cancer research and in the testing of drugs, cosmetics and many other substances. A rare case where Grey Goo actually turned out to benefit humanity.
Better yet, the cells are a Black Box; no one in decades of research has been able to figure out why they won't die! At this point in time, the mass of her post-death divided cells is greater than the mass of her body at death. Something on the order of 20,000 kilograms worth. And that's in the sixty years since she passed away.
But there is also a downside: HeLa cells benefited humanity but not Henrietta's impoverished and undereducated family; when they first heard about the immortal cells they thought scientists had her actual body being kept alive somewhere. When a woman wanted to talk to them about Henrietta for a book, they initially hung up on her because they thought she was another scientist looking to exploit them.
Then there's the theoretical threat of Strange Matter. The television series The Universe featured it in their 10 Ways to Destroy the Earth, and basically, everything on Earth melts like a warm ice cream sundae into non-organic goo. Sweet dreams!
There are far more energetic particle collisions happening every second as subatomic matter bombards the planet and occasionally smashes into the atmosphere or the ground. So far, in the last nearly five billion years, we've somehow managed to evade such a fate. This suggests that either the energy required to produce strange matter is far larger than what the LHC can manage or the odds of such collisions producing it are so incredibly small as to be considered nearly impossible. The Earth is safe. For now!
The scenario was first proposed by Eric Drexler. He has since dismissed it as unrealistic as its much easier to create a bunch of tiny dumb robots that receive their instructions from a central computer (that can be disabled if it malfunctions) than it is to create tiny robots that know and are capable of doing everything they need to make copies. The dumb nanobots should serve any purpose we'd need smart nanobots for so someone would have to really be trying to create a grey goo scenario.