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

Detailed walkthrough
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.

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 you persuade Bobbi to leave or betray her, Fahrenheit will reward you 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 you choose to side with Bobbi,so this path will yield the most profit as it allows you access to the Strongroom loot as well. Even if you 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.

Trivia

 * 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 you 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.
 * If you try to make Nick hack the terminal at some point, Sonya might get stuck and you 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 you without you breaking him out.
 * 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.