Mushroom War

The Mushroom War is the cause of the post-apocalyptic setting of the Land of Ooo. Not much is known about the war at the moment, but the inclusion of "Mushroom" in the title is probably referring to a mushroom cloud generated from a nuclear bomb. If so, then it is likely that the Land of Ooo and the creatures who inhabit it were created from the nuclear fallout that irradiated the land. This is further supported by the ruined pieces of modern technology scattered across the land, and the beginning of the opening theme (which briefly shows the remains of nuclear bombs and other weapons). It was confirmed by Pendleton Ward that Marceline and Ice King are survivors of the war due to their supernatural powers.

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.

Strangely, the Land of Ooo also contains technological devices that are far more advanced than what exists today, such as Universal Translators and Holographic Newspapers. It is unknown whether these are based on pre-war technology or are more recent inventions.

During full shots of the post-war earth, it is shown that a land mass similar to north and South America remain intact while the gigantic chunk of earth that is missing, was in the region of North Asia, which was the location of Russia. This suggests that the country was completely obliterated by nuclear weapons.

Evidence or clues of former civilization
In many episodes of Adventure Time, there are many hints or clues regarding the world and the civilization that existed prior to the Mushroom War, including as follows:
 * Adventure Time Theme Song: An opening shot showing various weapons and technological debris, including grayed land and undetonated nuclear missiles lying around, and an arm reaching upward from a tree trunk. There also appears to be the remains of a tank's tread in the center of the shot and a pink necklace and headphones. The bashed up television is another remnant.
 * In the "Animated Short", Pen's (Finn) mind is transported back in time to Mars. The world is shown full unlike the world shown above.
 * "Tree Trunks": Traffic signs, which have since somehow mutated into Sign Zombies. It is unknown how this has happened. The painting in Tree Trunks' house looks like it could be Egypt due to the triangular structures it shows that may be 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 Businessmen. 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. When the Businessmen, along with Finn and Jake, are shot up into the sky, they fall back down revealing the entire Land of Ooo. Ooo seems to be a large island, with open ocean surrounding it.
 * "My Two Favorite People": When Finn and Lady Raincorn 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 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 artifacts. Near the River of Junk, a car door can also be spotted close by. Also, Jake reveals he got his magic powers from a "magic" mud puddle. This puddle could have had radiation from nuclear bombs.
 * "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. You can also see the remaining tops of cars while they are floating over what might have been an ancient road. All this would seem to imply that sea levels have risen significantly since the war, which would also explain the world's altered Geography and the island-like nature of Ooo. There is also ruins of old pre-war buildings that can be seen when Jake tries to teach Finn to swim.
 * "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. In addition, the stolen chest of coin from the tower contains a cell phone in the bottom left corner.
 * "What is Life?": NEPTR is made from an old microwave, a device used by former civilizations from the past. Also, when Finn's looking for stuff to make NEPTR with, there is a bunch of other pre-war items.
 * "His Hero": In Billy's cave, TVs, a bathtub, and other items can be seen lying around.
 * "The Eyes": Jake plays Moonlight Sonata on his viola, which was written by Beethoven.
 * "Storytelling": There are many 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, suggesting that it may have previously been a body of water. Also, in the cave, a pack of what looks like zombified humans try to eat Finn and Jake. These zombies are probably mutated survivors from before the war.
 * "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 also 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. The radius of the bomb is outlined by the crater, explaining how the radiation spread around the entire planet.
 * "Death in Bloom": Debris of civilization, including numerous police vehicles, in the Land of the Dead. There are also crashed UFOs, possible hinting technology when the apocalypse happened was very advanced, or aliens. (UFOs were seen in Her Parents as part of the Rainicorn-Dog Wars but it is unknown if it is from that, and in the episode, Mystery Train, a poster on a passenger car reads "Watch Out For 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. Also, all the dead humans might have been killed by the war.
 * "Susan Strong": The Hyoomen Tribe live in the ruins of an underground city. 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. The inhabitants were possibly humans driven there during the war and later mutated. 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/mech arm sticking out of the debris, suggesting that perhaps robots/mech were used during the Great Mushroom War. There is also what seems to be the front of a tank sticking out of the garbage when Finn and Jake first jump down the hatch. Looking even closer, you can see artificial UV lights strapped onto the pillars, suggesting that the underground city may be, in fact, an underground "bunker city" of sorts where the humans would survive while the war waged on above their heads.
 * "Mystery Train": The train itself is most likely a relic from pre-war times.
 * "Rainy Day Daydream": Finn uses a nuclear missile. Since Finn and Jake most likely do not know how to make a missile, this may have been scavenged from pre-war times.
 * "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. Similar video cassettes and VHS tapes appear again as tapes of the Ice King's diary in the episode, Holly Jolly Secrets. The fact that Finn and Jake say they see the FBI warning on all the movies they "dig up" further explains past civilizations.
 * "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. The Lich gets it powers from "Toxic Waste". 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.
 * "Mortal Recoil": While it is unknown where they would get them, Jake implies that the Lich's "Well of Power", which he recreated in Princess Bubblegum's bathtub with various chemicals collected by Finn, is a vat of toxic waste left behind in his former lair. Also, 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 (Finn eating a sandwich and Jake spray-painting a picture of Lady Rainicorn) 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 "H" pad on top, and other tall buildings can be seen nearby.
 * "Morituri Te Salutamus": The destroyed arena is most likely 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 her home, then later catches her father eating her fries at a destroyed diner. This could be the fallout of the Mushroom War, which could cause a continuity problem, or it could be wreckage from a previous military conflict that eventually escalated into the Mushroom War. 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 near it. Also, a sewer pipe leaks out water for the moat around the castle.
 * "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 football stadium. Also, when the sun sets, in the background, there can be city buildings and towers from modern times in the overgrowth of the forest.
 * Legends of Ooo: A game on cartoonnetwork.com, mentions the Mushroom War. If you click on the radio in Marceline's house, Jake states that, "They stopped making these before the great Mushroom War."
 * "The Creeps": A portrait of what appears to be of New York City's silhouette.
 * "Beautopia": Beautopia appears to be the ruins of a luxury resort, as seen in the title card. When Susan, Finn, and Jake arrive at Beautopia, it seems as if it was some sort of 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 appear to be evil creatures inside floaties that survived during the war. That, or the floaties mutated.
 * "Thank You": Outside the home of the Snow Golem appears to be a snow covered bulldozer, which could possibly be an artifact from before the war. 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. This (and the academic worms talking about Nietzsche's Übermensch, along with Jake's mention of Mozart and Beethoven) indicates that not all knowledge pre-mushroom war is lost. There is also a limousine buried in the ground near the home of the Banana Man.
 * "Holly Jolly Secrets Part I": The place where Ice King buried his suitcase full of tapes looks like an old junkyard. Next to Finn was a boat that had a harpoon on the front of it. It was probably used for whaling. 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 a skeleton with a green hat, possibly an elf, a gag assumed to be put in as a reference to its holiday air date. 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. In the first clip Simon is seen in a photograph with a sniper in the background. Outside the window that Simon (the Ice King) is talking by, a city is seen, undamaged. Simon said he purchased the magic crown from a dock worker in Northern Scandinavia, a region of Europe. During his video, Simon states, "the power of the crown will save him with its frost." He mentions that he does not know what this means, suggesting that the video might have been recorded right before the beginning of the war. (*A sudden snowstorm is seen to start up outside the window shown in the tapes that quickly blankets the city. This could possibly be an allusion to the onset of a nuclear winter, or the onset of the crown's powers, creating the Ice Kingdom around him.) This may mean that the Ice Kingdom is the city. Simon also mentions that his former fiance, Betty, left him due to his irrational behavior caused by the crown. It is also possible to see a map of the eastern part of the Americas behind Simon in the first part of the video.
 * "Marceline's Closet": When Finn and Jake are playing Hide and Seek, ("Cloud Hunt") Finn sings a song that goes "Over the mountain the ominous cloud, coming to cover the land in a shroud, Hide in a bushel, a basement, or cave, but when the Cloud comes a' hunting, no one is safe!", maybe referring to mushroom clouds or nuclear fallout created by nuclear bombs in the Great Mushroom War. (or the fog of biological weapons, which could also help explain the very sparse numbers of humans) While they are hiding in Marceline's closet, the Spider that lives in Marceline's wall looks like it has many eyes, big fangs, and somewhat radioactive.
 * "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.
 * "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.
 * "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.
 * "Web Weirdos": The piece of wall that Jake and Finn bounce off of when doing stunts seems to have rebar. Rebar appears to not be very common among the residents of Ooo. When Ed goes to find Barb a present in the forest, he passes a derelict vehicle in the background and by the Dead Knight, you can see an electronic panel on the damaged wall to the left.
 * "Legends of Ooo": In The Forest where the Teenage Bear is found, one can see a car that says "Wash me."
 * "Daddy's Little Monster": Further reference to the Mushroom War is found as magnetic tourist plates stuck onto 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.

Timeline
We know that Marceline is 1000+ years old, but was born before the Great Mushroom War (as she is a child in Memory of a Memory) however, we also know that the Ice King is 1000+ years old, and was alive before the great mushroom war. Considering he seems to be somewhere between 20-50 in the tapes from Holly Jolly Secrets, we can presume, using these estimations that the Mushroom War occurred about 950 years before the episodes are staged.

It is unknown just when the Mushroom War took place. In the episode "Her Parents," it is stated that the Rainicorn-Dog Wars took place over "thousands of years." However, this does not prove when the Great Mushroom War happened, since the Mushroom War went on in the land that is now Ooo, but the Rainicorn-Dog Wars took place in the Crystal Dimension. Or alternatively Jake may have exaggerated for drama's sake and simply meant "a very long time" when he said that. In the same episode, Lady Rainicorn's father, Bob Rainicorn, says that he thought humans were extinct. The extinction would have been caused by the war, or the mutation of the humans changing their appearance due to the fallout. This would explain some of the various creatures that roam Ooo.

In "Memory of a Memory," Marceline appears as a young girl in a post-apocalyptic environment. In this scene, a burning city in the background suggests the Mushroom War occurred fairly recently, although plants growing in the wreckage of a house could suggest some time has passed since the calamity happened, but it has also been noted in nuclear testing and usage (e.g., Hiroshima) that the radiation encourages plant growth, and days after nuclear bombing occurs plants and flowers start to grow like weeds. Because Marceline has been confirmed to be approximately 1000 years old, this could mean that the Mushroom War took place just under 1000 years before the events of the series. It has been said that the present in Ooo takes place one thousand years in the future.

It could have happened sometime further in the future than the "present day" that the series is airing, i.e. much later than the early 21st century, considering the relics of more advanced technology they sometimes find.

In "Holly Jolly Secrets Part II," BMO puts on the last tape which shows Ice King as a young human, who tells the video diary about his fiancée Betty and a crown he wore that tells him secrets (Ice King's Crown). Throughout the video, he is shown looking more and more like Ice King. The early part of his video diary as a human show him living in what appears to be human civilization, either before or during the war. It is seen through the window that the city is suddenly covered in ice, possibly an the onset of a nuclear winter. This could possibly be the beginning of the Ice Kingdom. It could be determined that Ice King is at least as old as Marceline if both survived the war or humans where transformed into mutants.

Comics
In Issue 4, a still human 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.