Mushroom War

The Mushroom War (usually referred to as the Great Mushroom War) was an apocalyptic event that occurred roughly a thousand years before Adventure Time. The war resulted in the near extinction of the human species and left their civilization in ruins throughout the Land of Ooo.

Origins
Before the war, human civilization was prevalent throughout much of what is known now as Ooo. Although the cause of the war remains unknown, its name suggests that nuclear weapons were heavily used in the conflict. This is supported in "Holly Jolly Secrets Part II," where one of Simon Petrikov's video diaries shows a plane flying outside his window. Although the scenery outside his window indicates no sign of devastation, it is inferred that the war had begun and the aircraft was, in fact, a war plane. The beginning of the opening theme also suggests this as it briefly shows the remains of nuclear bombs and other weapons in Ooo. In addition, most episodes show the remains of various armaments and pre-war technology throughout the land.

The war likely ended when the mushroom bomb was dropped along with the other nuclear weapons. This is evident from the episodes "Finn the Human" and "Jake the Dog" when Finn wishes the Lich, a product of the mushroom bomb, to have never existed. When the wish is granted, Finn is transported to a world where Simon sacrifices himself to stop the mushroom bomb and freezes the entire Earth in the process. Despite some destruction, without the detonation of the mushroom bomb, humanity would continue to exist. Also, in "Finn the Human," Farmworld Marceline suggests that the mushroom bomb likely concluded the war as she states that the world would have been annihilated if Simon had not stopped the bomb with his ice powers.

The war likely ended approximately one thousand years before the events of Adventure Time. This is suggested from Marceline's age as she states that she is one thousand years old in the Farmworld timeline. Since she was a young child during the Mushroom War, roughly a thousand years had past since the conflict ended.

Aftermath
The end of the war left much of the world devastated and devoid of much life. The infrastructure that had existed throughout the land had been mostly destroyed. The little human infrastructure that survived, such as girder bridges and office buildings, now lay in ruin. A great deal of human technology also remained scattered throughout the land. Abandoned and destroyed vehicles were a common sight in the devastated landscape. Despite the great destruction, some humans managed to survive the war, such as Marceline, Simon Petrikov, and Moe. However, most presumably perished during the war. This is suggested in "Simon & Marcy" as there are no other humans in sight throughout the entire episode. Also, in "I Remember You," Simon says to Marceline that they may be alone after the apocalypse.

Some individuals survived the war through supernatural means. One example is Hunson Abadeer, Marceline's dad, who was seen in a flashback in "Memory of a Memory," showing him and Marceline right after the bombs were dropped in the war. It is likely that he may have been in the Nightosphere during the war. This is evident in Marceline's flashback in "I Remember You," as Hunson is nowhere to be seen while Marceline is crying while mouthing "Daddy." Therefore, it can be inferred that he abandoned or lost her sometime after the Mushroom War. Finally, in "It Came from the Nightosphere" Hunson Abadeer asks if Marceline turned the family axe into "Some kind of lute," hinting that he has spent an incredibly long amount of time in the Nightosphere.



Despite the destruction from the war, the land slowly began to recover and soon life was once again allowed to freely inhabit the earth. Only a few signs of the nuclear devastation remained in Ooo. One example was in the episode "The Vault," where Shoko helps Princess Bubblegum establish her new kingdom by covering up an irradiated river at the site of the Candy Castle. In the process of covering it up, Shoko becomes disfigured and mutated by the effects of the waste and appears as she had in premonitions in earlier episodes of the series. The episode implies that the Candy Kingdom is built atop a pond of nuclear waste.

Along with the rise of new civilizations, such as the Candy Kingdom, a diverse variety of intelligent life settled down in Ooo. Many were able to recover or develop technology for their own use. Examples of this are universal translator devices and holographic newspapers. It is unknown whether these are based on prewar technology (the year of the war was never stated, but we know that it must have happened sometime after 1982 because the Ice King is heard singing the theme song and making reference to the sitcom Cheers in the episode "Simon & Marcy" which premiered that year) or are more recent inventions. Despite this, most human inventions, especially those related to transportation, remained lost to the inhabitants of Ooo. In "The New Frontier," Jake mentions this when he says that "rocket ships haven't been reinvented yet," implying that the technology to build conventional space-faring vessels was lost during the Mushroom War.

The first mention of the war in continuity was in "Video Makers," when Finn and Jake discover government copyright warnings before the movies they watch for their film club. Even though the civilizations that created these warnings would have been long gone by their time (and that films bought for home video usage generally are protected to be used for private usage only, which being held in a home would be considered as such), they decide to abide by them to avoid breaking the law.

Evidence or clues of former civilization

 * To see a chronological list of episodes, see List of episodes.

In many (but not all) episodes of Adventure Time, there are hints or clues regarding the world and the civilization that existed prior to the Mushroom War, including as follows:

Season 1

 * "Tree Trunks": Traffic signs, which have since somehow mutated into Sign Zombies. The painting in Tree Trunks' house appears to be of Egyptian pyramids.
 * "The Enchiridion!": When Finn and Jake first walk through the forest of Mount Cragdor, pieces of metal and some dryers/washers can be seen as they look for the sacred book. Some of the pieces at the top, right corner of the path may be sinks.
 * "Business Time": Ancient artifacts frozen in icebergs, including computers, a bike, baby shoes and the Business Men. When the Businessmen try to remember where they came from, they seem to be in pain. Finn also uses a flamethrower built from two fuel tanks connected to a rifle by a tube. A tank is also seen later in the episode.
 * "My Two Favorite People": When Finn and Lady Rainicorn confront Jake and Tiffany, the bottoms of upturned cars can be seen in the ground. Jake also uses a military issue phone and headset, and some mutated and regular skulls can be seen near the fire pit where Jake, Lady Rainicorn, and Finn are sitting. The bottom of Lake Szelezon is covered by a horde of gold, and the area closest to the shore has the remains of a ship wreck on it.
 * "The Witch's Garden": The remains of a highway system and a plane's wing can be seen near Gary's nest; The River of Junk is essentially a river of mundane pre-war garbage. A car door can also be spotted near the River of Junk. When Gary picks up Finn a computer monitor can be seen with a tree growing in it. An airplane wing can been seen nearby Gary's nest. The Video recorder that the witch uses is probably from before the war.
 * "Ocean of Fear": Underwater city ruins and vehicles appear, and a brief scene shows a damaged aircraft carrier and a stranded tank. An even briefer scene depicts two dead bodies on a couch in the remains of a pre-war living room. Finn and Jake also take to the depths in a submarine which contains a fire extinguisher, which is most likely from before the war. One can see the remaining tops of cars while they are floating over what might have been an ancient road. There are also ruins of old prewar buildings and battleships that can be seen when Jake tries to teach Finn to swim. A prewar tank can be seen stranded on the building as well. The buoy that Finn stands and the wrench he uses to knock himself out are most likely from before the war.
 * "City of Thieves": When Finn and Jake are in Penny's tent, a double barrel shotgun can be seen sitting among a pile of gold coins, and a tommy gun can also be seen in the background. In addition, the chest stolen from the tower contains what appears to be a hand-held video game system in the bottom left corner. Also, right before the wizard vanishes a can opener can be seen on the tower next to him. The fishing rod used to take Penny's basket and the boots that Jake stole are probably also from before the war. When Finn falls into the King of Thieves' room a keyboard can be seen in the background.
 * "What is Life?": Neptr is made from an old microwave, a tin can, small tank treads, a circuit board and insulated wires.
 * "Henchman": At the start of the episode, Finn and Jake are playing on the remains of what looks like a dragon. In the foreground, an old wooden boat is buried in the soil, and in the background, the remnants of a castle can be seen.
 * "His Hero": In Billy's cave, TVs, a bathtub, and other items can be seen lying around.

Season 2

 * "It Came from the Nightosphere": The final battle between Hunson Abadeer and Marceline and Finn takes place on what appears to be an ancient battle field, covered in the ruins of an old castle, and scattered with the bones of dead soldiers. Finn uses one of these dead soldier's swords to fight Abadeer.
 * "The Eyes": Jake plays Moonlight Sonata on his viola, which was written by Beethoven. Jake himself mentions that it is written by Beethoven (mispronouncing it "Beet-hoven"), and also mentions Mozart ("Mot-zart").
 * "Storytelling": There are remains of radios and debris scattered throughout a forest. The young bear is also spraying graffiti on what looks like an overgrown van.
 * "Power Animal": A broken TV is seen beside the trunk of Finn and Jake's tree house when Jake walks up to the water girls.
 * "The Other Tarts": Various boats and whale skeletons can be seen in the Desert of Doom.
 * "The Silent King": The storyboard for "The Silent King" contains the first time The Mushroom War is mentioned by name in the series, heard when Gummy is reading from the rule book. However, this line was cut from the actual episode. The overturned auto mobile that appears during the goblin riot is another artifact from past civilizations.
 * "The Real You": A shot of Earth appears, and a huge chunk of it is missing. The missing piece of Earth could have been a result of the Great Mushroom War.
 * "Death in Bloom": Debris of civilization, including numerous police vehicles, in the Land of the Dead and crashed UFOs. They also take an escalator down to the entrance to the Underworld, and a TV is in the ground when they "sneak" past the entrance.
 * "Susan Strong": The Hyooman tribe live in the ruins of either an underground city, or a bunker that failed at its purpose. In the underground city, a car, a refrigerator, and some sort of submarines/huge ships can be noticed. In the storyboard, a warning reading "Biohazard Danger" appears on the hatch, although this was cut from the episode.Which can be proof that The Hyooman Tribe was once humans and they were turned to mutants by the radiation from the Mushroom War. Also, in one brief scene, the camera pans over a pile of junk. Upon closer inspection, one can see what appears to be a robot arm sticking out of the debris. There is what seems to be the front of a tank sticking out of the garbage when Finn and Jake first jump down the hatch.
 * "Mystery Train": The train itself is most likely a relic from pre-war times. Parts of the locomotive, its passenger cars, and cars interior have a style like those of trains of the 1800s. Just as Finn and Jake are boarding the train, a railroad "X" crossing sign and the remnants of a former second track can be noticed.
 * "Go With Me": The Drive-in theater houses the remains of many aged cars.
 * "Belly of the Beast": The remains of a damaged car rest in the throat of the monster as Finn and Jake return to confront the bears in the stomach.
 * "Video Makers": Video cassettes feature movies such as Heat Signature that were made prior to the war. Jake even mentions the Mushroom War in the episode. The fact that Finn and Jake say they see the FBI warning on all the movies they "dig up" is further evidence they are of pre-war origin.
 * "Mortal Folly": The base of the Lich's power is a ruined subway station with deserted trains and littered with human skeletons scattered on the floor as if they had been killed that very moment around the station. Character designer Andy Ristaino has stated that these skeletons "were supposed to feel like they were east coast commuters ... all bundled up for winter." Outside the station is what appears to be a nuclear missile. There are also ruins of buildings in the background. the subway and its platforms are like those in New York or New Jersey.
 * "Mortal Recoil": When Finn is telling Jake what he was giving Possessed Princess Bubblegum he says Plutonium, which was used to make Nuclear bombs.
 * "Heat Signature": The title card shows Finn and Jake standing on a tearing, fading billboard which stands in a destroyed urban background. In front of Marceline's cave there is a defunct highway that acts like an awning. There are also the remains of an old car across from the cave entrance. Later, the ghosts take Finn and Jake to the top of what looks like a torn up skyscraper with a helicopter crashed on the roof. The skyscraper does have a helicopter pad on top, and other tall buildings can be seen nearby.

Season 3

 * "Morituri Te Salutamus": The destroyed arena is most likely The Colosseum in Italy from before the Great Mushroom War.
 * "Memory of a Memory": Many of Marceline's childhood memories appear to be around the ruined remains of civilization. She is seen as a little girl living in a ruined city with a derelict tank near houses, then later catches her father eating her fries at a destroyed diner. In Marceline's memory of a little girl, the unknown city in the background can be seen on fire. Around Ash's house, there are many tires and other debris strewn about the area. There is also what appears to be a mobile home on the roof of his house, which has an aged tow truck balanced on top.
 * "Hitman": Outside the Breakfast Kingdom is a desert with an abandoned railroad car and an abandoned dump truck with haystack near it.
 * "The Monster": When Finn and Jake find the villagers, various metal tools are on the ground.
 * "Wizard Battle": The arena for the battle seems to be a broken down sport stadium. Also, when the sun sets, city buildings and towers from modern times in the overgrowth of the forest can be seen in the background.
 * "The Creeps": A portrait of what appears to be of a city's silhouette. Another painting of a woman with the date 8/18/12.
 * "Beautopia": Beautopia appears to be the ruins of an underground shopping mall or luxury resort due to signs left on the sides of buildings such as "Eat," "Sale," "Buy" and "Food." Also, the Lub Glubs live inside pool floaties.
 * "Thank You": Outside the home of the Snow Golem appears to be a snow covered bulldozer. The house in which the Snow Golem lives could also be a remnant from before the war, as suggested by Tom Herpich's promo art for the episode, which depicts a family of humans (or humanoids) living in the house in a time before it was covered with snow.
 * "The New Frontier": When Finn and Jake see a rocket built or found by Banana Man, Jake states that rockets haven't been re-invented yet when Finn is worried he might die. There is also a limousine buried in the ground near the home of the Banana Man.
 * "Holly Jolly Secrets Part I": When Finn first stumbled upon the Ice King collecting his tapes, there appears to be a ruined backhoe in the distance. The location that the Ice King buried his suitcase full of tapes looks like an old junkyard. Next to Finn is a boat with a harpoon on the front of it. A truck, a car, and a small plane also lie near the Ice King and Finn. When Finn and Jake are digging for the suitcase, a shoe is hanging from a tree branch and a TV is seen underground where the suit case was buried. In this hole is also a skeleton with a green hat. A trumpet is also seen tied to a tree branch above the Ice King. The video camera Ice King used to record his diary most likely dated before the war as well.
 * "Holly Jolly Secrets Part II": The last tapes played showed the Ice King as a human (Simon Petrikov). The Ice King's crown apparently turned him into what he is now, as seen in the videos. An undamaged city is seen outside the window that Simon is talking in front of. Simon said he purchased the magic crown from a dock worker in Northern Scandinavia, a region of Europe. It is also possible to see a map of the eastern part of the Americas behind Simon in the first part of the video.
 * "Paper Pete": At the end of the episode when Finn and Jake are walking away from the library, one can see buildings rising from the ground surrounding the library. The library itself seems to be made before the Mushroom War, too.
 * "Marceline's Closet": Near the beginning of the episode, Finn sings a nursery rhyme about the war when playing hide & go seek.  The lyrics are "Over the mountain, the ominous cloud, coming to cover the land in a shroud. Hide in a bush, a basement a cave, but when cloud comes a-huntin' no one's a-safe."
 * "Ghost Princess": The Cemetery looks like it was made before the Mushroom War, and has a lot of modern-day objects in it. Multiple cars and a passenger train car can be noticed in the cemetery. The Squeez-E-Mart appears like a convenience store of the former civilization formerly run on wind turbines and solar panels that have since become derelict.

Season 4

 * "Five Short Graybles": In the study of the Story Teller is a holographic "globe" of Earth (similar to the globe of Earth usually used in study images), with a massive gash over a corner of the world just like Earth's depiction in "The Real You." It also seems that Finn's catapult for his "super high five" is partially made of some sort of military device.
 * "Web Weirdos": The piece of wall that Jake and Finn bounce off of when doing stunts seems to have rebar. When Ed goes to find Barb a present in the forest, he passes a derelict vehicle in the background and by the dead knight, one can see an electronic panel on the damaged wall to the left.
 * "Daddy's Little Monster": There are tourist plaques hanging from Hunson Abadeer's kitchen cabinets. The plates are for Vegas, Orlando, and Arizona.
 * "In Your Footsteps": Ruined buildings can be seen rising out of the forest Finn and Jake were walking around at night.
 * "Princess Monster Wife": A picture of a city in winter can be seen in the Ice King's room.
 * "Hug Wolf": At the beginning of the episode, there are some ruins near the Tree of Blight.
 * "Card Wars": The card game Card Wars uses technology that seems to be from before the Mushroom War.
 * S4e15 Earth.png"Sons of Mars": At the beginning of the episode, an abandoned double-tracked railroad line is shown. On the left track, an out-of-service rail car is seen. Later, Magic Man's derelict house is surrounded by many vehicles, which are to be abandoned. Also, Earth is shown from space showing the hole on its side. Abraham Lincoln offers Death a penny in exchange for Jake's soul. The penny's production date is "20XX D" (minted in Denver); it also says "IN XXD WE TRUST" ('X' meaning the letter or number was covered up by moss).
 * "Burning Low": When Princess Bubblegum is trying to explain why Finn cannot be with Flame Princess, on her hologram a picture of the earth can be seen that looks similar to the one on "Five Short Graybles." During Jake's dream about being promoted to president, the remains of a highway and a plane wing can be seen; however, since this was in a dream, this might not be canonical.
 * "Lady & Peebles": During one of her Korean conversations, Lady Rainicorn mentions Vietnamese noodles, which refer to the former existence of the nation of Vietnam. Princess Bubblegum also uses GPS, a system developed by the US government in the 1970's.
 * "Who Would Win": There are broken ship hulls and other ship parts strewn around the lake.
 * "I Remember You": The Ice King is seen to have a US Highway 101 sign in his "Past Room" as well as an image of a comet hurtling for Earth. During the song, "Remember You," clips are shown: what appears to be a destroyed city, and a street corner where Marceline is crying, when Simon Petrikov comes and takes a teddy bear, Hambo most likely, from a destroyed toy store and gives it to her. Earlier in the episode, Marceline shows the Ice King a newspaper clipping of when he was still Simon Petrikov. The US 101 sign from the Ice King's Past Room is seen again in the destroyed city, which can reason that Marceline and Simon's meeting occurred on the Pacific Coast of the United States along the 101 corridor between Washington State and California.
 * "The Lich": For a brief second, one can see the planet Earth when Jake makes the Enchiridion map of the multiverse enhance. The blown up chunk of the planet is visible. Earth is depicted as the second planet from the sun, because after 1,000 years, the sun has grown to consume Mercury, therefore making Venus the closest planet to the sun.

Season 5

 * "Finn the Human": Scenes from the Mushroom War are shown, such as Simon Petrikov on a hill staring at war planes, a destroyed city with the Lich and other mutant creatures walking against a green mushroom cloud, and Simon freezing a nuclear bomb-like weapon before it detonates.
 * "Five More Short Graybles": Finn and Jake discover an old book of nursery rhymes, which they believe to be a spellbook. The Story Teller also mentions at the end of the episode that "nobody has had five fingers for 20 blabillion glaybles."
 * "Up a Tree": When Finn and the Squirrel are flying off on a Frisbee, there is a shot of a destroyed highway in the background. Finn and Jake are also playing a modified version of dog fetch.
 * "All the Little People": The hill Finn and Jake are sitting on at the beginning of the episode appears to in fact be a collapsed building that is covered with grass and ivy. Also, the area surrounding the hill seems to be the ruins of a human city, as at least one other ruined skyscraper can be seen as well as the ruins of a highway system. This landscape was seen in the world of the mini-people at the end of the episode, with the same ruined buildings and highway.
 * "Jake the Dad": When Mr. Fox shows the Baby Eating Fox and the Babies book to the other foxes, some of the foxes are resting on what appears to be an old abandoned car.
 * "Mystery Dungeon": In the second room they enter, it looks like a pre-war kitchen with destroyed tile floors and an oven.
 * "All Your Fault": In Princess Bubblegum's lab, there is a mobile of the solar system, and the giant hole in the earth can be seen.
 * "Vault of Bones": While Flame Princess and Finn walk through the woods to find a dungeon, there is the remains of an old car buried in the undergrowth in the background.
 * "The Great Bird Man": Various boats and whale skeletons can be seen in the Desert of Doom. This location was previously visited in the episode "The Other Tarts."
 * "Simon & Marcy": The majority of this episode takes place 996 years ago, a few years after the war, with having a mostly destroyed city. There appears to be mutated creatures that ooze slime out of pores from their bodies, and a pink, sentient bubblegum-like substance taking over parts of the city. Also, when Simon and Marceline walked to the "Clambulance," they seemed to have been walking on a dry river, greatly resembling a part of the Los Angeles River.
 * "BMO Lost": Throughout the episode, the background of the forest is filled with ruins, such as cars, chairs, and bottles. When BMO first lands, Bubble emerges from an old cup. Bubble's accidental Rube Goldberg machine ends with a tree falling on the skeleton of a long-dead hunter. BMO and Bubble discuss Ricky's departure by what appears to be an old pipe system.
 * "James Baxter the Horse": When cheering people up, Finn and Jake come across a sad ghost on a potentially abandoned railroad.
 * "Princess Potluck": When Ice King goes to the forest, there appears to be the remaining top of a red car sticking out of the ground.
 * "The Suitor": In the Vapor Swamps, one can see a few decomposed buildings in the background.
 * "The Party's Over, Isla de Señorita": When the Ice King first hears Island Lady's singing, ruined buildings can be seen in the water off the coast of the island, similar to the ruins seen in "Ocean of Fear."
 * "Another Five More Short Graybles": The DVD's that the Ice King has seem to be from before the Mushroom War due to the fact that they have human actors. The house that Mr. Fox lives in might be from before the war, as indicated by how old and easily the walls were broken down. Also, the engine Finn and Jake use for their time machine looks like it was manufactured before the war.
 * "Candy Streets": around the train tracks near the Candy Train Station, there are remnants from what appears to be a previous rail system, as well as the wreckage of other train cars.
 * "Be More": The Mo Co. was built over 1,000 years before the show takes place, before the war. Moe was also a human from before the war, although his body parts were replaced with robot parts, making him only part human.
 * "The Vault": When Shoko is riding up to the mansion, various wrecked buildings can be seen littering the country side.
 * "Love Games": The entrance to the Slime Kingdom, which is located near the Bad Lands, is littered with sunken ships, crashed airplanes, bones, and what appears to be a mid-20th-century space capsule.
 * "Red Starved": The entire episode takes place in an underground city either made of sand or transformed into sand. Some old structures can be seen. Marceline taps a soldier made of sand with a spoon, causing him to fall apart. Finn estimates the ruins to be over 500 years old, so its possible that the ruins date to after the Mushroom War.
 * "We Fixed a Truck": The episode focuses on a truck that Finn found in a forest. The truck, nicknamed "Hot Daniel" is later fixed by Finn, Jake, BMO and Banana Man and appaers to be a 1954 Chevrolet 3100, thus making it pre-war.
 * "The Pit": Joshua, and later Finn and Lady Rainicorn, explore a decrepit dungeon. However, the time frame for when the dungeon was created is not known, and it is possible that the ruins date to after the war.
 * "James": While traversing through the Desert of Wonders, various ruined buildings and highway systems can be seen in the background. There are also some tanks, a missile stuck on the ground, and what appears to be the wing of a military plane with a propeller.
 * "Apple Wedding": Lumpy Space Princess emerges from the remains of an old well. In the background is what appears to be an abandoned house.
 * "Blade of Grass": The opening shot shows ruined buildings and the remains of what looks like either a bridge or a road system. When Finn is running towards the Forest to get rid of his cursed sword, the remains of what appear to be a plane turbine are seen sticking out of the ground.
 * "Rattleballs": The episode takes place in a junkyard, in which the scattered remains of pre-Mushroom War materials seem to be located, such as sofas, wrecked cars, and nuclear weapons.
 * "The Red Throne": as Flame Princess and Cinnamon Bun are fleeing the Fire Kingdom through the forest, several wrecked cars can be seen in the background.
 * "Betty": Simon creates a portal that jumps back to the time immediately before the Mushroom War to say goodbye to Betty. During Marceline's monologue, the episode flashback to scenes immediately following the war, showing wreckage of a city. When Marceline, Finn, and Jake rush to the Ice Kingdom, they pass by what appears to be an old snow plough.

Other

 * Legends of Ooo: A game on cartoonnetwork.com, mentions the Mushroom War. If one clicks on the radio in Marceline's house, Jake states that, "They stopped making these before the great Mushroom War." In the Forest where the Teenage Bear is found, one can see a car that says "Wash me."
 * Adventure Time Theme Song: An opening shot showing various weapons and technological debris including undetonated nuclear missiles, the remains of a tank's tread, a pink necklace, a television and headphones. There is also what appears to be a zombie arm reaching upward from a tree trunk.  And when the screen goes past Marceline's cave a driveshaft and two axles can be seen to the right.

Timeline
In "Memory of a Memory" Marceline appears as a young girl in a post-apocalyptic environment. A burning city in the background suggests the Mushroom War had affected that area fairly recently, although plant overgrowth suggests that some time has passed since humans inhabited the area. Marceline is one thousand and three years old, suggesting that the Mushroom War occurred roughly one thousand years into the show's past. This theory was proven when, in "Finn the Human," Farmworld Marceline says that Simon Petrikov saved the world from the Bomb of Death (mushroom bomb) 1,000 years ago to Farmworld Finn. In "Holly Jolly Secrets Part I" and Part II BMO plays tapes which show Ice King as a young human. The Ice King, then known as Simon Petrikov, narrates a video diary about his life. The early part of his video diary shows him living in what appears to be human civilization, either before or during the war.

A scene in "I Remember You" ends up revealing a memory of Simon traveling through the destroyed world and meeting an alone and crying Marceline. Simon calms her by giving her a toy. During the episode, it had been revealed that they might be the only survivors to the Mushroom War. However, this is unlikely due to the fact that Finn appears to be a normal human boy, meaning some humans may have survived the Mushroom War, though it is not known whether any besides Finn are currently alive.

The alternate universe of "Farmworld," created by Finn in an attempt to stop the Lich from existing, further seems to reveal the possible birth of modern Ooo. Although the Mushroom War still occurred, a massive nuclear-like weapon was fired by one of the sides. Though the weapon did not hit initially due to the sacrifice of Simon Petrikov, it ultimately did finally hit the Earth a thousand years later. When it hit, a massive green energy similar to that of The Lich emerged and covered the entire world quickly. It can be assumed that when this "bomb" hit Earth in the actual universe, this began the emerging of the Lich, the Human Apocalypse, and the end of the Mushroom War; leading to the beginning of the transformation of the entire world into what would emerge, over the next thousand years, as Ooo; a world where technology would exist side by side with magic.

Comics
In Issue 4, a still unchanged Marceline can be seen hiding in a cave as mushroom bombs bombard the planet around her, creating the giant gash on the side of the planet. In the collected edition of the comic book, Marceline's coloring in the flashback is changed to the more demon-like look seen in "I Remember You."

In Issue 13, Randall N. Byron reveals that the first Mushroom War happened 1,002 years ago.

In issue two, after Finn and Jake kill the Lich in outer space, Finn sees Earth and wonders if the hole from the Mushroom War had been there all along.

In "Marceline And The Scream Queens," Marceline mentions to Princess Bubblegum how Bubblegum thought Iggy Pop was a soda. This means that the mushroom war was after the 80's/90's, and that they knew each other at the time.

Trivia

 * In an interview with Pop Candy, Pendleton Ward said that the post-apocalyptic aspect of Ooo was not in the original concept of the series, but emerged as a result of the episode "Business Time," when the writers decided the Business Men being frozen in ice made the world post-apocalyptic, and "we just ran with it."
 * It is possible that Goo Monsters could be former humans mutated by the Mushroom Bomb, theorized by the abundance of the creatures in former highly populated areas, such as the metropolis in the episode "Simon & Marcy." Also, they seem to leak a very similar green goo as the irradiated pools of waste in the same episode aforementioned, as well as the episode "The Vault," it draining out of most of their orifices, such as their ears, eye sockets and mouth. They are later seen again in "James," wherein James himself turned into one.

In other media

 * In the YouTube animated show Bravest Warriors (created by Pendleton Ward), a quick frame shows Earth, with a chunk missing from it, seen when Chris is imagining the future.