Mushroom War

The Mushroom War (usually refered to as the Great Mushroom War) is the cause of the post-apocalyptic condition 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 the mushroom shaped explosion generated by atomic weapons. 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). Marceline and the Ice King are the only known characters to have survived the Mushroom War.

In "Finn the Human" Farmworld Marceline states the Mushroom Bomb was dropped 1000 years ago. In Farmworld, Simon Petrikov froze the Bomb in midair, though it came so close to contact that it crushed him. Current Marceline is also 1000 years old. This means that in the normal Universe of Adventure Time the Bomb would have also been dropped around 1000 years prior to current events.

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 prewar technology (the year of the war was never stated, it could take place centuries after our time) or are more recent inventions.

In "I Remember You," the Ice King is shown to have a drawing of earth with a giant meteor headed towards it, possibly resulting in the missing chunk- although this may be referencing the asteroid in the episode Wizard.

Evidence or clues of former civilization
In many 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:
 * 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." 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.
 * "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 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.
 * "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 garbage. A car door can also be spotted near the River of Junk.
 * "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 see the remaining tops of cars while they are floating over what might have been an ancient road. There is also ruins of old prewar 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 chest stolen from the tower contains what appears to be a hand-held video game system in the bottom left corner.
 * "What is Life?": Neptr is made from an old microwave, a tin can, small tank treads, a circuit board and insulated wires.
 * "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. 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 it's 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. 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. 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.
 * "Rainy Day Daydream": Finn uses a nuclear missile.
 * "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.
 * "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.
 * "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 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 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 8/20/12.
 * "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": 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. 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. 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.
 * "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." 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, you 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.
 * "Loyalty to the King": Lumpy Space Princess was holding a sound amplifier, and the Ice King had a disposable razor.
 * "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 house is surrounded by many vehicles. 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."
 * "Lady & Peebles": During one of her Korean conversations, Lady Rainicorn mentions Vietnemese 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 you could see the planet Earth when Jake made the Enchiridion map of the multiverse enhance. The blown up chunk of the planet was visible.
 * "Finn the Human": Scenes from the Mushroom War are shown, revealing humanity at war without being sparked by supernatural occurrences. However, it is hinted that a massive nuclear-like weapon possessing energy tied to the Lich ended up overwhelming all sides and creating the land of Ooo. When Simon Petrikov prevents this bomb from hitting Earth, history is changed evolving the planet into the Farmworld universe.
 * "Five More Short Graybles": Finn and Jake discover an old book of nursery rhymes, which they believe to be a spellbook.
 * "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.

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 approximately one thousand 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 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 is a normal human boy, meaning a very small number of humans 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, the Lich emerged 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 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. During this issue, the hole in the planet is filled by Desert Princess using sand from the Lich's Bag of Holding, but it is unknown if this change will carry over into the TV series. 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."

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 Businessmen being frozen in ice made the world post-apocalyptic, and "we just ran with it."