Mushroom Bomb

The Mushroom Bomb first appears in "Finn the Human" and is likely one of the last weapons of mass destruction dropped during the Mushroom War. Its main effects were the creation of the Lich and his Well of Power, and led to the Land of Ooo's creation.

It is unknown exactly why the Mushroom Bomb was dropped, though it was probably due to the culmination of fierce hostilities between two or more pre-Mushroom War people, or nations.

Appearance
The Mushroom Bomb has the appearance of a stereotypical nuclear bomb, similar to that of the real-life "Fat Man" bomb, detonated in WWII.

The bomb's explosion cloud is also similar in shape to those created by real-world nuclear weapons, with its distinctive mushroom shape. However this cloud is different in that it glows a deep green and is filled with many skull shaped howling souls.

Trivia

 * In the Farmworld Timeline, The Mushroom Bomb remained dormant, encased in ice and guarded by Farmworld Marceline for over 600 years (She and the rest of the earth were unfrozen 400 years after the bomb was frozen, 1,000 years after it was first dropped), but was eventually detonated by "Finn Mertens" when he was acting uncontrollably with "Ice King's Crown."
 * The bomb leaves behind a crater filled with a glowing green substance that granted Farmworld Jake an increase in size, glowing green eyes, torn lips, devil horns and a lust for death like the Lich. In Farmworld Marceline's flashback, a silhouette very much like the Lich's is seen emerging from the Mushroom Bomb's cloud surrounded by demons. As the Lich's stated goal is to end all life, it might be carrying on the Mushroom Bomb's purpose of destruction.
 * There is a slight inconsistency about the size of the crater created by the bomb. In the episode "Jake the Dog," the bomb only detonates up to the size of an house, while the lasting effect in the real timeline is a large crater covering a portion of the Earth. The reason behind this is that the bomb is stale for a thousand years.
 * It sends off clouds of green skulls like the Lich's Well of Power, possibly implying magical properties. This implication is reinforced by the fact that the Farmworld had no magical elements beyond the presence of Farmworld Marceline and the Ice King's Crown until the bomb detonated. She recalls that the bomb was poised to bathe the Earth in "mutagenic horror," implying that perhaps it contained magics that mutated the Earth's inhabitants into the beings seen in the Land of Ooo.
 * The cave holding Simon Petrikov's body and Farmworld Marceline resembles the subway station where the Lich had his Well of Power in the original timeline.