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 it is possible that the "Mushroom" in the name is a reference to a mushroom cloud generated by 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, the beginning of the opening theme (which briefly shows the remains of nuclear bombs and other weapons), and mutated Shrimp seen in the episode, "Ocean of Fear," which could have been caused by radiation from the bomb. It was confirmed by Pendleton Ward that Marceline is a survivor of the war due to her supernatural powers.

The first mention of the war in continuity was in Video Makers, when Finn and Jake discover government 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 prevent from breaking the law. This proves that there was no knowledge of an actual film industry with it wiped out with the war and the end of civilization as it was.

More about the war will be discussed in Season 3.

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 debris of destruction, including grayed land and undetonated nuclear missiles lying around, as well as a decaying hand reaching out from a log.
 * Tree Trunks: Traffic signs, which have since somehow mutated into Sign Zombies. The reason how this has happened though is unknown as traffic signs are not alive and therefore cannot mutate.
 * Business Time: Ancient artifacts frozen in icebergs, including baby shoes and the Businessmen. Finn also uses a flamethrower built from two fuel tanks connected to a rifle by a tube.
 * 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
 * Ocean of Fear: Shrimp that have mutated beyond their original form, underwater city ruins and vehicles and a brief scene showing a damaged aircraft carrier and a stranded tank. Finn and Jake also take to the depths in a submarine.
 * Storytelling: There are many remains of radios and debris scattered throughout a forest. Finn seems to notice some of this, but quickly forgets about it.
 * The Other Tarts: Various boats can be seen in the Desert of Doom, suggesting that it may have previously been a body of water. Considering the background rock structures, and the large marina in a desert area, it's very likely that this may have once been Salt Lake City, considering its proximity to the Desert of Doom (Nevada) which is bordered by the Kingdom of Fire (California)
 * The Real You: A shot of Earth, only with a huge part of it missing.
 * Death in Bloom: Debris of civilization, including police vehicles, in the Land of the Dead.
 * Susan Strong: An underground shelter where exists a tribe of "humans" that have been mutated beyond their original form.
 * Go With Me: The Drive-In Theater houses the remains of many cars.
 * Belly of the Beast: The remains of a car rest in the throat of the monster as Finn and Jake return to confront the bears in the stomach.
 * Video Makers: The survival of various video cassettes featuring movies that were made prior to the war.
 * Mortal Folly: The base of The Lich's power is an abandoned, 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. Outside the station is what appears to be a Nuclear Missile.
 * Heat Signature: In front of Marceline's Cave there is a torn up highway that acts like an awning. There is 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.

Trivia

 * If Marceline's admission of being "a thousand years old" as stated in the House Hunting Song in "Evicted!" is correct, then it can be estimated that the Mushroom War occurred at most a thousand years ago. We may be able to further narrow this range by estimating Marceline's perceived age (which could be late teens to early twenties) and subtracting this from a thousand; The reasoning behind this would be because vampires traditionally stop aging once they are turned and that she obviously was not turned after the war or she would not have survived (although this may be pure speculation, as at least a few humans have survived post-war, as proven by the existence of Finn). However, with Marceline's tendency to fib about her life and accomplishments, this can only be taken as theory as opposed to truth.
 * The storyboard[1] for the episode The Silent King contains the first mention of The Mushroom War in the series, heard when Gummy is reading from the rule book. However, this line was cut from the actual episode.


 * It is unknown what year the mushroom war took place. The show might take place way in the future because modern day technology is scattered everywhere. Abraham Lincoln is well known in the show and he was born in a year between the 1800s and 1900s. This also unidentifies the date when Marceline was born.
 * It is also possible that the Mushroom War took place much longer than thousands of years ago, as in the episode Her Parents it is stated that the Rainicorn-Dog wars took place "For thousands of years" but the wars could have been going on at the same time, seeing as how "the great mushroom war" went on in the land that is now Ooo and "the rainicorn-dog wars" took place in the crystal dimesion.