|Home world:||Planet Earth (Nebula M78 Universe)|
|First appearance:||Ultraman Ace Episode 29: "TThe Sixth Ultra Brother"|
|Height:||2 ~ 52 m|
|Weight:||180 kg ~ 44,000 t|
Subtitle: Underground Superman (地底超人 Chitei Chōjin?)
- Height: 2 ~ 52 m
- Weight: 180 kg ~ 44,000 t
- Origin: Underground
Undergroundmon wanted to invade the surface world for conquest. He used the chouju Gitagitanga to destroy industrial areas and help block out the sun. TAC was quick to respond to the situation and Undergroundmon that his species needed more space than they have underground. The next day, Undergroundmon summoned Gitagitanga to destroy more industrial areas. He trapped the sixth Ultra brother and proceeded to join the chouju. He grew to enormous size are downed TAC's planes. Hokuto summoned Ultraman Ace and Ace fought against the duo. The two monsters overpowered Ace and the sixth Ultra brother called the Ultra star to encourage Ultraman Ace. Ace destroyed Gitagitanga with the Ace Lifter. Enraged, Undergroundmon fired his laser but Ace set him on fire with a Ultra Beam. Groaning, Undergroundmon collapsed then exploded.
- Voice actor: Osamu Saka
- It is easy to notice the similarities between Undergroundmon and another alien Gironman, who also used a monster to fight Ace and Zoffy. In fact, Gironman's suit was modified for Undergroundmon.
Powers and Weapons
- Size Change: Undergroundmon can change his size at will.
- Claw Hands: Undergroundmon can use his claw hands to bash enemies in combat.
- Mouth Restrainers: Undergroundmon can emit energy restrainers from his mouth.
- Torso Beam: Undergroundmon can fire an energy beam from his torso.
- Eye Beams: Undergroundmon can fire red beams from his eyes.
|Ultraman Ace Kaiju & Seijin|
|Verokron | Yapool | Chameleking | Vakishim | Garan | Aribunta | Giron Man | Brocken | Alien Metron.Jr | Doragory | Muruchi II | Gammas | Zaigon | Alien Antira | Unitang | Sabotendar | Barabas | Ace Killer | King Crab | Cattle God | Cowra | She-Devil | Hotarunga | Black Pigeon | Android Couple | King Kappa | Zemistlar | Aprasar | Aprasar Fairy | Space Mask | Black Satan | Giant Yapool | Mazaron Man | Yojo | Mazarius | Alien Orion | Sphinx | Alien Hipporito | Lunaticks | Undergroundmon | Gitagitanga | Red Jack | Baktari | Coakes | Bad Baalon | Kaiteigagan | Dreamgillas | Soundgillar | Machless | Snowgiran | Namahage | Alien Fire | Firemons | Alien Steal | Kaimanda | Shishigoran | Iceron | Woo II | Fubigirara | Onidevil | Gasegon | Daidarahoshi | Hanzagiran | Verokron II | Yapool Woman | Univerlages | Aquarius | Alien Revole | Signalion | Geegon | Alien Simon | Jumbo King|