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Before the Great War, the bunker was used as a secret base of operation for the Defense Intelligence Agency as can be seen written on the floor in the main hall. It was later discovered by the Railroad, who re-purposed it as their headquarters. They also uncovered there the Predictive Analytic Machine, a pre-War advanced robot designed to predict the future with extreme precision and accuracy, using the most complex formulas, methods, and algorithms based on the data provided.
It wasn't long before the Railroad's enemy, the Institute discovered the location. Synths were later dispatched to wipe out the Railroad, forcing them to relocate to the Old North Church, leaving the bunker in the hands of the Institute.
The entrance is unmarked but can be easily found when looking for a drainpipe overlooking a puddle between the Drumlin Diner and Lexington map markers. The exit is an elevator in the basement of a Slocum's Joe, a pre-War donut shop west from Super Duper Mart. Entry can be gained either by hacking a master locked terminal yourself at any time or by having Deacon do it for you during the Tradecraft quest. It is not necessary to be on the quest in order to obtain the perk magazine.
Clearing the location takes you on journey through utility tunnels until a hole in the wall provides access to the bunker. All the Railroad agents you encounter are dead, but some of their defense systems still function and can be activated to help fight the synths that have invaded. After working your way though the opposition, you can restore power to the basement elevator to allow an exit trough Slocum's Joe
If Tradecraft is active, Slocum's Joe will be guarded by synths, turrets and mines. You may want to clear them out from afar before entering the drainpipe to avoid being cornered in the donut shop basement later on.
- Two fusion cores. One in the lower level generator room, near where the sewer entrance empties into the building, one right near the end, behind a locked security gate, just before the elevator that leaves the area.
- A U.S. Covert Operations Manual in the central area flanked by two sets of staircases. Go up the stairs, it'll be on the right side of the desk. The room it's in overlooks a computer-filled work area.
- Carrington's prototype - in the room off to the side of the vault.
- Two hazmat suits can be found on a table on the second floor, in the room where Carrington's prototype is located.
- One mini nuke on a shelf to the right of Carrington's prototype.
- Two Stealth Boy`s on a metal shelf with the prototype
- Deliverer, a silenced pistol obtained during the Tradecraft quest.
- Jangles the Moon Monkey in the "Databanks" section of the bunker on top of a computer terminal.
- In the databanks room is a poster which shows mixed and matched robots. This seems to foreshadow the Automatron DLC.
The Switchboard appears only in Fallout 4.
Behind the scenes
One of the terminals in this area lists DEFCON reports, and suggests that it may have been the Chinese that were the first to strike during the Great War (terminal specifically states "poss." as in "possibly"). The last entry shows that the system went offline at 09:47 hours, the same time shown on most of the clocks in the Commonwealth and the Capital Wasteland.
If you use the terminal in the main hall underneath the office with the U.S. Covert Operations Manual you might get stuck in between the chair and the desk. Saving and reloading will fix this.
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