Enter Nukapedia's Pumpkin Carving Contest - going on now!
The basement armory is an unmarked location on the Island in 2287. The entrance can be found near a fuel station southeast of the radiant crest shrine, locked behind a Novice level lock.
This small room contains a chemistry station in order to work out the recipe of Vim Captain's Blend, a terminal to organize notes on sabotage, and a weapons workbench and firing range to hone the skills needed to intimidate Vim distribution drivers.
The basement armory appears only in the Fallout 4 add-on Far Harbor.