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 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.

Edit: You'll need to talk to one of the drifter people to unlock Quest: "Talk to Bobby" prior you can talk to her.

Bugs

 * When breaking Mel out of the Diamond Security Office, after talking to him and leaving the building; he will reappear outside with you without you breaking him out.