The Big Dig

In the back alley behind Kleo’s weapon shop, there’s a metal door. Approach it and Bobbi the ghoul will offer work for you. After agreeing, this quest will become available.

Head inside Bobbi’s place. In the second floor, there’s an advanced terminal which can be used to open the nearby safe. Meet Bobbi in her basement then join the other two already digging. After reaching the tunnel workers, they’ll encounter mirelurks then will quickly retreat from the area. Head to the next room and exterminate the mirelurk razorclaws. Talk to her again and she’ll invite you to talk to her by the noodle shop in Diamond City.

Fast-travel to the meeting location in Diamond City and talk to her. She’ll then ask you to get her tech man, Mel, out of jail. You can either bribe, threaten, or simply use your Charisma to let Mel out. If you’re trying to talk your way out of it, make sure to save before doing so. Once he’s out, head out of the prison and talk to Mel.

Now travel back to Goodneighbor and enter Bobbi’s house. Head to the digsite. After Sonya, Mel's modified eyebot, blasts off a portion of the wall, explore it. You’ll encounter mirelurks while exploring so be prepared. Also, you can manually order Sonya to blast off any fragile wall you encounter by interacting on the said wall.

In the first room you’ll explore, there will be two fragile walls. Destroy the wall in the NE first to Lead to a dead-end room. There’s a Deep Mirelurk King there. Kill it and loot the duffel bag and access the free Power Armor inside.

Blast the next fragile wall and continue forth. More mirelurks will appear so clear the corridor then blast the next door. You’ll eventually reach a train station. This location is filled with several ghouls so be prepared for an ambush. There’s an Advanced terminal and door in the NW side of the station, with a cooking station as well. Go around the train to the NE to reach the fragile wall. Destroy it and head to the next area. More ghouls are waiting for you here and more radioactive barrels. Once you cleared the area, destroy the next fragile wall.

In the next room, there’s an Expert Safe. Clear the next room of ghouls then loot the Mr. Handy wreckages in the room. The next fragile wall is located in the bottom of the room.

In the end of the tunnel, there will be two exits. One leads to a room to a basement area to the SW with a Fusion Core behind an Expert security door. The real path is to the NE, inside a room behind a security door and a chem station. You can unlock the security door using the terminal in the same room. More ghouls are waiting for you here. Clear them out.

There’s another security door in the lower floor which can be opened by the Expert terminal beside it. The next fragile wall is next to a broken security door with a green steamer trunk. Break through the wall then talk to Mel in the next room. Once done, break the next wall and talk to Bobbi. Leave the room and let Mel have Sonya blow up the ceiling. Continue up the ramp then enter the strongroom.

Farenheit, Hancock’s right-hand woman/ bodyguard will be waiting for you here. She’ll reveal that you’ve broken into Hancock’s storeroom in NH&M Freight Depot, not Diamond City as Bobbi stated. You can try to persuade Bobbi to leave. If that fails, you can either betray her or stick with her. If you did what is right, you can kill Bobbi then talk to Farenheit to receive the unique mini-gun, the Ashmaker. This will complete the quest. Since you stayed in Hancock’s good side, you can head back to Goodneighbor and pay your respects to him personally.

If you sided with Bobbi, you'll have to kill Farenheit and the Triggermen with her. You can loot the Ashmaker from her body.

After talking to Hancock, you’ll be able to recruit him as a new companion. If you've chosen Bobbi, Hancock will ask you to kill her first before you can get him as a companion.