Location from Gravity Falls by Alex Hirsch / Disney Channel
An underground research station built by Ford and McGucket beneath an artificial tree in the Gravity Falls Forest — designed as a lab, a fallout shelter, and a place to contain things that should not be let out.
Ford wanted a secure laboratory. McGucket, secretly terrified of what their work was producing, wanted a sanctuary from the weirdness of Gravity Falls and designed the bunker as both. He hired lumberjacks to speed construction, then used the memory gun to make everyone forget. Ford didn't find out until later. The bunker's most significant resident was the shape-shifter — a creature with the ability to mirror any DNA it encountered, kept in cryogenic containment for study. By the time Dipper and Mabel find it in Season 2, the shape-shifter has been there for thirty years and has opinions about this.
Accessed through a hatch beneath an artificial tree in the forest. Underground: a functional fallout shelter with decades of supplies, cryogenic containment tubes, and a research lab. The shape-shifter's cage is here. Emergency lighting, metal walls, a layout that looks like someone expected to survive something very bad.