The Big Dig

The Big Dig is a quest in Fallout 4.

Quick walkthrough

 * Meet Bobbi inside
 * Join the dig
 * Exterminate the Mirelurks (0/3)
 * Talk to Bobbi
 * Meet Bobbi in Diamond City
 * Get Mel out of jail
 * Recruit Mel
 * Meet the gang in the Dig
 * Dig to the Strongroom
 * Meet with the gang in the basement.
 * get out of the basement
 * Enter the Strongroom
 * Side with Bobbi or Fahrenheit

Detailed walkthrough
The player character receives the quest after talking to a woman named Bobbi 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's no distinguishing characteristics. Breaking him out with lockpicking will likely result in the security becoming hostile.

Finally, the team goes into the tunnel, traveling trough 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 is 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 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. However Ashmaker can also be acquired from Fahrenheit's corpse if player character choose to side with Bobbi,so this path will yield the most profit as it allows the Sole Survivor access to the Strongroom loot as well. Even if the player character kill Fahrenheit and side with Bobbi, John Hancock will still be available as a follower after a dialogue at the entrance to Goodneighbor.

Quest stages

 * I met Bobbi No-Nose in an alley in Goodneighbor and agreed to work for her. She asked me to work on digging a tunnel in her basement.
 * Bobbi promoted me after I exterminated a group of mirelurks in her basement. She asked me to meet her in Diamond City to tell me more about the job.

Behind the scenes

 * The quest name is a reference to The Big Dig, a massive public works infrastructure project constructed in Boston that involved the building of various highway tunnels under the Downtown area.

Bugs

 * Trying to talk to Bobbi through the door in the miscellaneous quest 'Talk to Bobbi' wont work, so the Sole Survivor can't obtain the quest 'The Big Dig.'
 * After killing the guards, Bobbi and Mel will become unresponsive and the quest will not be able to be completed.
 * Sometimes Mel can be made permanently aggressive to the player, making it impossible to destroy the ceiling below the Diamond City Strongroom.
 * If the Sole Survivor tries to make Nick hack the terminal at some point, Sonya might get stuck and the Sole Survivor won't be able to continue the quest.
 * When breaking Mel out of the Diamond City security Office, after talking to him and leaving the building; he will reappear outside with the Sole Survivor without breaking him out.
 * Past The Dig location underground, objects may float and bounce unnaturally, especially after Sonya's blasts.
 * 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.
 * Mel may disappear along with Sonya during the quest making optional walls unable to be destroyed. activating quest related walls will make them reappear.
 * When entering The Strongroom, the option to open the door may disappear, making it impossible to enter.