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. Finally, the team go into tunnel, travelling trough metro stations and basements. After they made it underneath the strongroom and blowing the floor it reveals that it's not Diamond City's strongroom but the storage room owned by John Hancock, Goodneighbor's mayor. Player character has a choice of persuading Bobbi to leave, kill the guards, or betray Bobbi and kill her instead.

Detailed walkthrough
If you persuade Bobbi to leave Fahrenheit will reward you with the Legendary Minigun Ashmaker.

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.