|This article is about a/an location in Hikari Sentai Maskman.|
The Underground Castle (地底城 Chitei-Jō?) is the headquarters of the Underground Empire Tube. It is the royal palace notably associated with Earth Emperor Zeba, who meets with his minions within a central court. It also has connection to various means of imprisonment, most notably the "frozen crypt" where those trapped within are instantly frozen with little chance of being released. It also possesses a library for research .
Zeba's ultimate plan involves raising the castle as he darkens the surface world, allowing for Tube to control both the surface and the underground. Using his power, he creates an illusion castle that is enough to darken the surface until the Maskman are able to complete their Aura Power training and dispel the fake castle using Great Five.
In the final phase of Tube's invasion, Zeba decides to physically lift the castle to the surface, using a dangerous mechanism that has to be controlled by hand and with the guarantee to kill anyone who operates it. As the castle rises, it distributes the Evil Particles of the underground to instantly coat the surface in darkness. The Maskmen combine their five aura with a pendant manipulated by the former underground princesses Ial and Igam, destroying the castle and changing Zeba into his true form as Lethal Doggler II to be finished by the heroes.