In Building 21, the Generator Room is an important area to find if you want rare chests, or if you want to get the second Insured Weapon Slot in DMZ. Like anything else in this sector, the map is hidden and you have to navigate the dangerous corridors to find your prize.
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Where is the Generator Room in Building 21 DMZ?
Located on the eastern side of the map, between A1 and B1, the Generator Room can be found on the first level of Building 21. Neither A1 nor B1 have a front entrance to this cramped room, so you need to look for the side entrances that are part of the cramped corridors instead.

The Generator Room in DMZ contains a few rare chests in the center of the room, as well as a safe that you can drill into. When you finish the Black Mous Tier 1 quests, all you have to do is step right into the middle of the room and your DMZ mission is complete. Once you leave the area, you will have access to the second Insured Weapon slot.