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 can't carry the box. After Book tells that she had arms, Ice Cube was happy because she, Book and Match can escape the LBUF. And after they all escaped, the trio continues their journey finding the rest of the FreeSmarters and escape from Evil Leafy.
- Inside the Lego Brick's Underground Factory, there's a warning sign reads "Warning, beware of warning signs!"
- It is unknown how or when Lego Brick did build this factory inside Evil Leafy.