Mass Fusion Building

The Mass Fusion Building is a location in the Commonwealth This building is one of the Commonwealth's most recognizable landmarks due to its height.

Roof
The rooftop is an exterior location and has its own fast travel marker, the Mass Fusion executive suite. It is normally only accessible as part of the main quest endgame. The Mass Fusion executive ID that allows access to the elevator down will not be present if no relevant quest is active.

Above glass floor
This section, starting on the eighth floor, is accessible from the roof by a quest-locked elevator. The area is mostly comprised of destroyed offices, with multiple caved-in floors creating one large interior space. This area will be abandoned if visited before Spoils of War or Mass Fusion (by using a jet pack power armor mod to fly up from the lower floors). During these two quests, the Sole Survivor will be ambushed by synths or the Brotherhood of Steel as they exit the elevator.

Below glass floor
This area includes the building lobby and some walkways and offices. It is accessible from the front door, and is occupied by Gunners if encountered before Spoils of War or Mass Fusion. When approaching from above while on one of the relevant quests, the battle with the opposing main quest faction will continue here.

Reactor room
The reactor room is below ground level; the Beryllium Agitator sought by both the Brotherhood of Steel and the Institute is located here. After dealing with area security, including turrets and Sentry Bots, the agitator can be found in a large, highly radioactive central room.

Main building

 * Overdue book - On the fourth floor, on a desk in the northern section; one will have to go up to the fifth floor then back down to the fourth floor to conveniently access this section.
 * Strength Bobblehead - On the fifth floor, on the head of the giant statue.
 * Mini nuke - On the fifth floor, in the room to the northwest with the Gunner commander.
 * Full set of power armor - On the fifth floor, worn by the Gunner commander.
 * Mass Fusion labs key - On the eighth floor, on a desk in the northwest office.
 * Two Stealth Boys - One is on the eighth floor, on a metal shelf next to the chemistry station. The other is in the same room as the Freefall Legs, on top of a small bookshelf.
 * Tesla Science Magazine issue #5 - On the ninth floor, on a computer bank in the southeast.
 * Fusion core - Five floors from the top in a small room through a destroyed wall to the south, on a desk. Only accessible using a jet pack.
 * Freefall Legs - Three floors from the top in a small room through a destroyed wall to the east, inside a Novice-locked safe. Only accessible using a jet pack.
 * Jack Rockford's log - In the same room as the Freefall Legs, in the intern's terminal. Only accessible using a jet pack.
 * Executive key and ID card - In the executive suite, inside the desk. Only accessible during Mass Fusion or Spoils of War.

Reactor

 * Unique damaged hazmat suit - In a drawer right outside the decontamination room that leads into the reactor room.
 * Two hazmat suits - One is in the locker room behind the attendant's terminal, sitting on a shelf of one of the lockers. The other is next to the analyst's terminal, on top of a safe.
 * Fusion core - A Sentry Bot with the core spawns behind the northern security door, which can be opened through the Master-locked reactor security control terminal on the south side of the room before the reactor room. Alternatively, one can take the core and trip the security; the doors will then open automatically.
 * Overdue book - On a desk.

Appearances
The Mass Fusion Building appears only in Fallout 4.

Bugs

 * The can chimes on the first floor do not disappear after collecting them, and one can repeatedly obtain XP and tin cans for activating them again.
 * The bathroom scale trap on the first floor provides XP and materials upon disarming it, but the Tesla trap will still be triggered even after disarming the scale.
 * Taking the large circular elevator from the lobby back up to the Mass Fusion executive suite sometimes causes the player character to arrive in the wrong location in the exterior cell - a location which was not intended to be accessible.