The Underground Empire Tube is a collection of territories and underground tribes ruled over by the united Tube Empire. They were initially ruled by the Igam Family until they were overthrown in a coup by a mysterious figure named Zeba, who promptly declared himself "Emperor".
Under Zeba's reign the Tube became a militaristic society bent on invading the surface world. After Zeba was defeated by the Maskmen, Princess Ial, a descendant of the original Tube royal family, retook the throne and went to restore Tube to its original pacifist state.
Under the reign of the Igam family, the Tube were kept mostly evil. Years before Zeba's revolt, a rogue Earth Imperial Beast named Lethal Doggler terrorized the underground until the forces of the Igam family hunted it down and killed it, restoring peace to the underground.
However, years later the Igam family would be overthrown in a coup by a mysterious figure named Zeba. The Queen of the royal family managed to discover Zeba's true identity and make a recording of it, but she was killed by Zeba before she could expose it. After declaring himself Emperor of the Tube, Zeba began building up Tube's military forces for an invasion of the surface world. Curiously, while he murdered the rest of the Igam family, he kept the two oldest children of it, Igam and Ial, alive and even appointed Igam his second-in-command. Igam served under Zeba in hopes of succeeding him as ruler one day and taking back the underground for the Igam family.
While Igam worked closely with Zeba, his sister Ial was sent to the surface as a spy. The Tube eventually began their invasion of the surface world but their invasion was anticipated by Commander Sanjurou Sugata, who formed the Maskmen to fight against them.
Tube is comprised of several different tribal societies of underground humans and creatures, all united under the banner of the Underground Empire. Among the civilizations that are a part of the Tube united kingdom are the Fu Clan, a tribe of ninjas pledged to serve the imperial family of Tube, as well as the Baluga Tribe, a group of warriors who make up part of Tube's military. The Tube are a monarchist society ruled by a single king and queen, or later emperor during Zeba's reign. Also inhabiting the underground are a variety of monsters dubbed "Earth Imperial Beasts" who serve the Tube monarchy. While some roam freely, most are kept sealed in an icy room and thawed out to perform a specific task.