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. In level 33 of BFDIA 5b, Lego Brick trapped Book and Match into his underground factory for killing him (in level 31) and trapping him in an inescapable box (to get to the next level in level 32). In level 42, Book and Match found Ice Cube also got trapped in the underground factory too. Ice Cube explained that if she can carry a wooden box close to him at the exit, she could escape the factory. But due to her lack of arms, she couldn't carry the box. After Book tells her that she had arms, Ice Cube was happy because she, Book and Match can escape the LBUF. After they all escaped, the trio continue their journey of finding the rest of the FreeSmarters and escaping from Evil Leafy.
- Inside the Lego Brick's Underground Factory, there's a warning sign which 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.
- It is unknown how or when Lego Brick did build this factory inside Evil Leafy.
- This was the second underground factory shown in BFDIA. The first is Golf Ball's in Zeeky Boogy Doog.