The Big Dig

The Big Dig is a quest in Fallout 4.

Quick walkthrough
Player character receives the quest after talking to a woman named Bobbi in backstreet at Goodneighbor. She says that she's planning on breaking into Diamond City strongroom located under mayor office. At first 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 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 ghouls. 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 its floor 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 mayors 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.

Detailed walkthrough
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.

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

 * 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.