The Big Dig

The Big Dig is a side quest in Fallout 4.

Detailed walkthrough
The player character receives the quest after talking to a woman named Bobbi No-Nose in a backstreet at Goodneighbor. She says that she's planning on breaking into the Diamond City strongroom located under the mayor's office. At first the player character needs to go down in The Dig, a tunnel starting at Bobbi's house basement and help diggers fight mirelurks. After, she asks the player character to meet her in Diamond City and help free another member of the heist team, Mel.

Freeing Mel can take one of several forms: bribing the guard nearest to him, pickpocketing a key, setting the protectron behind him loose to kill the guards, or passing a moderate or hard persuasion check. It may be difficult to find the right guard as there are no distinguishing characteristics. Breaking him out with lockpicking will likely result in security becoming hostile. To the left of the cell is the office, inside steal the key from the desk, then sneak to the right side of the cell. When the guards are not looking unlock the cell with the key and meet Mel outside. If the guards spot you they will become hostile.

Finally, the team goes into the tunnel, traveling through old metro stations and basements filled with feral ghouls and more mirelurks. Mel will frequently express his doubts about the tunnel actually leading to Diamond City, as the team is heading too far South. After they make it to a cavity underneath the strongroom and blow the ceiling with the help of Sonya, a modified eyebot, it is revealed that it's not Diamond City's strongroom but a secluded storage room owned by John Hancock, Goodneighbor's mayor.

The room is filled with the mayor's henchmen. The player character should be subsequently given the choice to persuade Bobbi to leave and surrender, kill all the guards and stay with Bobbi, or betray Bobbi and kill her for the mayor instead; if the player character enters the room wearing headwear incorporating a targeting hud, Fahrenheit and her man will be automatically hostile, forcing the PC to kill them to continue. If you still have the headwear on after finishing the quest by getting paid, Bobbi will also immediately turn hostile and attack.

If the player character persuades Bobbi to leave or betray her, Fahrenheit will reward them with a legendary minigun with incendiary bullets (+15 fire damage); Ashmaker. Additionally, John Hancock will become available as a follower.

Even if the player character kills Fahrenheit and sides with Bobbi, Ashmaker can be looted from her corpse and John Hancock will still be available as a follower after finishing Hancock's follow up quest of killing Bobbi.

Behind the scenes

 * The quest name is a reference to The Big Dig, a massive public works infrastructure project in Boston that involved the building of various highway tunnels under the Downtown area. The project was famous for its record breaking cost overruns and the death of a commuter shortly after one of the tunnels fully opened to traffic.  Some of the soil removed during construction was deposited on Spectacle Island
 * Four wooden blocks spelling the name "Gary" in reference to Vault 108's infamous Gary clones are found in the subway car at the metro station.
 * During the confrontation between Bobbi and Fahrenheit, Mel will say "Dammit Bobbi", which is possibly a reference from the animated television show "King of the Hill". In the show, the main character Hank Hill continuously utters this line to his son Bobby.

Bugs

 * Bobbi's 3rd floor terminal in Bobbi's place may not allow you to sit down to hack, resulting in the safe being impossible to open
 * Trying to talk to Bobbi through the door may not work, preventing the Sole Survivor from continuing the quest. This can be solved by using console command and entering 'setstage ms16 15' to forcefully skip this stage of the quest. If this does not work then replace 15 with 20, 25 or 30. To open the door that requires a key to unlock, use console command 'unlock' (It may be hard to "target" the door in the console, try using "setscale 0" to verify you have the door targetted and not a dust cloud, "setscale 1" to undo).
 * One of the mirelurks during the "Exterminate the Mirelurks (0/3)" part may get stuck behind the first destructible wall and be unreachable, thus making it impossible to continue the quest. Try to leave the room, talk to Bobbi, and re-enter. Alternatively, reloading an earlier save before entering this stage of the quest will fix it. If you're playing on PC, you may use console command 'tcl' to toggle clipping off, walk through the wall and kill the mirelurk. Use 'tcl' again to turn clipping on again.
 * Mel might be released just when you enter or leave the Diamond City security building, without the player having undertaken any action to free him.
 * After pickpocketing the guards and freeing Mel, Mel may become unresponsive and the quest will not be able to be completed. This can sometimes be fixed by reloading multiple times, and waiting for Mel to leave the security office before the player.
 * Sonya may occasionally get stuck in walls or objects during fights, terminal accesses and other events, preventing her from removing walls. You can walk into her to move her, reload an earlier save or leave and re-enter the tunnels to get her out again. If playing on PC, you can click her and enter 'moveto player', or, if she can not be clicked, first use 'prid 11fc2e' to teleport her to you.
 * Sometimes Mel can be made permanently aggressive to the player, making it impossible to destroy the ceiling below the strongroom.
 * Mel may disappear along with Sonya during the quest making optional walls unable to be destroyed. Activating walls related to the quest will have the walls reappear.
 * When asking to break a wall, Mel will call Sonya but it will just make a quick turn and stop completely, no matter what. Saving, quitting and then reloading fix this.
 * When trying to break the final wall, activating it will make Mel glitch and unresponsive. Shoot Mel and then talk to him to resume the quest.
 * When trying to break down the final wall, it is not able to be knocked down. Instead, talk to Bobbi No-Nose to knock it down.
 * If this does not work, you can also use the "disable" command in the console to disable the wall. Alternatively, using "tcl" in the console will allow you to walk through the wall. The NPCs will follow you through.
 * When entering the strongroom, the option to open the door may disappear, making it impossible to enter. Reloading your game from the main menu usually solves this problem.
 * After killing the guards, Bobbi and Mel will become unresponsive and the quest will not be able to be completed. Reloading an earlier save may resolve this issue.
 * At several points in the strongroom, Bobbi (and sometimes Mel) or Fahrenheit and her men may suddenly become hostile. The cause may lie with the targeting HUD power armor upgrade. To prevent this from happening, get out of the power armor before entering the strongroom.
 * John Hancock will ask you to "Confront Bobbi" if you sided with Bobbi No-Nose and killed Fahrenheit during The Big Dig side quest upon returning to Goodneighbor. Upon arriving at Hawthorne Estate, she will ignore the player, thus preventing the player from obtaining Hancock as a companion.
 * When siding with Bobbi No-Nose she may become unresponsive after opening the train cart and only say "Let me check it out first" without ever going to it. To fix it just leave the safe house and re-enter it.
 * Hancock might initiate his dialogue apologizing for making the Sole Survivor confront Bobbi No-Nose without completed the quest to begin with.
 * Sometime Mel will disappear or Sonya will get stuck, simple workaround run back to the entrance and stare at any wall for a bit and they will reappear/unstuck.
 * On the last door, Mel and Sonya tend to break, and will not remove the wall.

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