The Motherlode

The Motherlode is a side quest in Fallout 76. This quest can be discovered while completing explore Hornwright Industries.

Detailed walkthrough
There are two ways to enter the building. When getting close, there will probably be some mutants and Scorched killing each other. You can clean up the outside of the building and work your way to the top entrance or there's an entrance on the bottom floor that enters into the lobby. In the lobby of the Hornwright Industrial HQ is an office belonging to Yvette Wiesman. Using the terminal and accessing the archived message dated 9.29.77 will finish the explore mission and begin The Motherlode. The water on the floor is highly toxic. The player character will be instructed to search Penny's (Penelope Hornwright - Sr. Ops Exec) office for more information. Her office is about five floors up. There is also an elevator door in this room that the player character will return to. In Penny's office upstairs, there is another terminal to use. Access the archived message dated 9.30.77 to update the mission.

Now the player character will need to acquire an ID card before they can use the elevator in the lobby to reach the upper floors. Go down the stairs one floor, then into the first door on the left. In the back, there is a computer which controls the Hornwright hiring system. Apparently, in order to get an ID card, the player character will need to become an employee. The player character will need to find a resume and submit it but taking the Hornwright senior executive exam is completely optional.

Another terminal in the same room hosts the exam for those wanting to complete the exam. There are five questions on the exam and all must be answered correctly (Number 2 has two correct answers, so either one will work). The answers are in the gallery below.

The next room has a desk where one can find the Senior Executive Exam Updates holotape. Play the tape to update the mission. The exam answers are now available from tech support, one floor down. The sheet is laying on a printer in the office. Go down two more floors (one up from the lobby) to the company archives room and use the terminal inside. Access the human resources database and then Penny's resume file. This will add her resume to the player character's inventory. Go back up to the third level and into the testing room again. Deposit the resume into the resume receptacle.

The connecting room has the ID card printer on top of a file cabinet. Activate it to gain an ID card. Now go back to the lobby and use the elevator to reach the executive floor. Look for a door with a blue laser grid. These are safe to walk through. Activate the external connection system terminal. The computer will request a repair beacon. The location for this information is not marked but is in the room. Look for a set of computers behind the ECS terminal up a short set of steps. Interact with the research terminal and access the research notations, then power struggles. This will give the player character the beacon code (36984).

Use the keypad to put the code into the security box behind the research terminal. This will dispense a repair beacon in the machine to the left. Activate the dispenser to gain the beacon. Leave the room and go to the opposite laser walled room and find the elevator, then take it to the "sub room." The beacon launcher is standing in very toxic water. Use a radiation suit, or Rad-X if needed, to get to the device and activate it. Move out of the way as a drill machine will enter the room through the floor. After it leaves, access the motherlode at the docking station dispenser in the same room. This will end the mission.

Rewards

 * Iron ore x2
 * Black titanium ore
 * Aluminum ore
 * Iron ore x2
 * Bottlecap x150
 * Silver ore
 * Plan: Combat rifle

Behind the scenes
Motherlode 003 is likely a reference to The Venture Bros's AI character M.U.T.H.E.R. with both using the same voice, sharing the same name, and asking for their creator upon waking up.