The Beast of the Underground is strange creature is an Easter Egg that appears when Nico offers an old bar of chocolate to the Khan's ghost in the London Underground. The action opens up an exit to the left of the entrance area near a pile of wood which would obstruct that exit usually. However, when the aforementioned conditions have been met a cutscene occurs.
Nico walks up to Robert Foster (protagonist of Beneath a Steel Sky) and warns him not to continue down the tunnel they are currently standing in as there is a monster lurking behind the walls waiting for them. Being the hothead he is, Robert ignores Nico's advice despite her saying that she's played this scene before and continues walking. At that point a giant tentacle slides through the wall and wraps around his torso and crushes him before dragging him through a crack. He is presumed dead.
- Despite the horrifying scene unfolding before her eyes, Nico's remarks that "they don't make animations like THAT anymore" and carries on with her quest.
- Robert Foster and the monster appears as an easter eggs in the London Underground Tunnel and is a referrence to the 90's adventure game by Revolution Software