Bunker

The bunker is an underground location the author of journals 1, 2, and 3 used as both a hideaway and base of operations for his experiments.

History
Over thirty years prior to the series, the author of the journals grew fearful he was being watched by forces who would seek to use the information he acquired for nefarious purposes, and created the bunker as a means of carrying out his work in secrecy. Securing it beneath an artificial tree within the Gravity Falls forest, the author used the bunker as both a fallout shelter and research lab, containing various experiments within it, including the  Shape Shifter, whose DNA possessed the power to constantly metamorphose.

Growing too dangerous to be kept free, the author fashioned him a metal cage to temporarily imprison him as he designed a device capable of cryogenic stasis, but was forced to abort his plans when the creature broke free. Sealing the bunker to prevent the Shape Shifter from gaining freedom, the author's fate was left unknown, and his base would lay dormant beneath the forest for years to come.

Dipper first finds the bunker's exterior after coming across its artificial tree in " Tourist Trapped ," eventually locating the security mechanism used in hiding Journal 3. Intrigued by its content, Dipper retreats to the  Mystery Shack  to reveal his findings to  Mabel.

The bunker's interior is unveiled in "Into the Bunker," where Dipper, Mabel, Soos and Wendy explore the location after learning of it in Journal 3. Their investigation is cut short, however, when the Shape Shifter attempts to destroy them, leading them into a skirmish that ultimately lands him as the author intended; frozen in cryogenic stasis. Despite their failure to learn the author's true identity, Soos inadvertantly brings his laptop out of the bunker alongside them, allowing their quest to continue on.

Exterior
The bunker is a subterranean complex built beneath the Gravity Falls forest, only accessible through a mechanical, technological automaton in the guise of a tree. A small chamber is used to house a device serving as both a surveillance system and operator to conceal Journal 3 in an underground crevice. A lever on the topmost section of the tree disguised as a branch activates the lowering of a spiral staircase, as well as the opening of a large door into the bunker.

=== Bomb shelter ===

The stairs lead to a small room with weapons, food stock, and Smez. A poster conceals a small hallway leading to another small room.

Security room
A chamber composed of tiles on the walls, floor and ceiling. Stepping on a specially marked tile in the middle triggers a booby trap in which the tiles individually expand forward to crush whoever is inside. The only way to prevent this is to press four specially marked tiles to open a door to safety.

Observation deck
A room linked to a number of security cameras in the lab, most likely meant to serve as a way to safely observe and experiment on the test subjects.

Hidden lab
A laboratory where all the experimental subjects are kept under lock and key, preferably in cryogenic stasis.

Creature den
A section of tunnels dug out by the Shape Shifter to live in, a swath of cloth over the entrance serves as a doorway.

Trivia

 * The bunker appears to function as both a fallout shelter and a research lab, as the author had stored supplies for a disaster that would last over 60 years and conducted various experiments with creatures such as the Shape Shifter.