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 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, and mutated Shrimp were 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.
 * Tree Trunks: Traffic signs, which have since mutated into Sign Zombies.
 * Business Time: Ancient artifacts frozen in icebergs, including baby shoes and the Businessmen.
 * 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.
 * 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.
 * The Real You: A shot of Ooo being similar to Earth, only with a huge gash like part of it is 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.
 * 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 died that very moment around the station.
 * Heat Signature: In front of Marceline's Cave there is a torn up highway that acts like an awning.The is also the remains of an old car across from the cave entrance

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. However, with Marceline's tendency to fib about her life and accomplishments, this can only be taken as theory as opposed to truth. (Why couldn't she have turned into a vampire in 1600?)
 * 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.