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:
The first shot of the opening sequence
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.
“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…”
Image: Hieronymus Bosch. The Garden of Earthly Delights [detail]. Oil on panel triptych, 220×389 cm. Collection: Museo del Prado, Madrid.