The interdimensional rift is a byproduct of the activation of the Interdimensional Portal.
The rift is first referred to near the end of "A Tale of Two Stans," when Ford tells Stan that while he and the twins stay in his home for the summer he will remain in the basement to try and contain any remaining damage caused by the portal's reactivation.
In "Dungeons, Dungeons, and More Dungeons," Ford explains to Dipper that, as he suspected, when Stan reactivated the portal, the instability of its use created an interdimensional rift. Ford contained the rift and hid it in a cabinet in his underground laboratory. He was the only person aware of its existence until he told Dipper, who he made promise not to tell anyone, not even his sister.
In "The Last Mabelcorn," the rift reappears in Ford's dream, with Bill Cipher claiming he cannot protect it forever. It is revealed that if Bill obtains the rift, he can use it to physically enter the mortal world. Later in the episode, Dipper tries to protect the rift from Ford, due to his belief he is working with Bill or possessed by him.
In "Dipper and Mabel vs. the Future," Ford and Dipper travel to a UFO buried beneath the town, planning to use some alien adhesive to keep the orb containing the rift from shattering. They are successful in finding it, but before they can apply the adhesive, Mabel, heartbroken that Dipper is thinking about staying in Gravity Falls as Ford's apprentice, runs out of the Mystery Shack, accidentally taking Dipper's backpack with the rift inside. Bill, while possessing Blendin Blandin, tricks Mabel into giving him the rift by telling her he can use it to stop time, fulfilling her wish for the summer to never end. Once Mabel gives him the rift, Bill smashes the containment orb under his boot, causing a gateway to the Nightmare Realm to open in the sky. From here onwards the rift appears over the sky.
In "Weirdmageddon Part 1," Ford explains to Dipper that, due to the rift emerging, Bill's world is spilling into theirs, and every minute Bill's power grows stronger. The rift is seen above the Fearamid, during the episode.
In "Weirdmageddon 3: Take Back The Falls," the rift can be seen again over the Fearamid. After Bill's defeat, the rift sucks Bill's Henchmaniacs and evey creature that came to Gravity Falls back to the Nightmare Realm, closing itself afterwards and undoing Bill's chaos, returning Gravity Falls to his definition of "normal."
In "Gravity Falls: Lost Legends," during the tale "Don't Dimention It!", is revelated that after Weirdmageddon, a bunch of residual rifts have opened across the whole town and Ford and his family go to seal them. Mabel falls through one when a monster drags her, forcing Stan and Ford to save her, at the end of the adventure, Stan and Ford seal the rift while chanting "Pines! Pines!"
The rift itself has no definite shape, moving around on its own. The inside looks like space and glows a faint blue with various stars of differing brightness, size, and shades of blue and black. It is contained in a snowglobe-like container with caution tape at its base. The orb containing the rift is secured with pipe-like supports, colored in a very dark grey.
- 212. "A Tale of Two Stans" (mentioned)
- 213. "Dungeons, Dungeons, and More Dungeons"
- 215. "The Last Mabelcorn"
- 217. "Dipper and Mabel vs. the Future"
- 218. "Weirdmageddon Part 1" (mentioned)
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