Lego Brick's Underground Factory was an underground factory owned by Lego Brick that is shown in BFDIA 5b. It is located under a house inside Evil Leafy. The factory houses numerous malicious traps with the intent of making a victim die repeatedly, as well as a statue of Lego Brick himself. In level 33 of the game, Lego Brick trapped Book and Match in his underground factory for killing him and trapping him in an inescapable box (in levels 31 and 32). In level 42, Book and Match found Ice Cube also got trapped in the underground factory. It is unknown how she got there.
- Inside the factory, a sign reads "Warning, beware of warning signs!"
- This may be a reference to Golf Ball's Underground Factory, which also has a warning sign with the same text.
- Grupheetie can be seen in the background of the factory, similar to Golf Ball's Underground Factory.
- It is unknown how or when Lego Brick built this factory inside Evil Leafy.
- This was the second underground factory shown in BFDIA. The first is Golf Ball's in Zeeky Boogy Doog.