Subtitle: Underground Monster (地底怪獣 Chitei Kaijū)
- Height: 89 m
- Weight: 62,000 t
- Origin: Cyberspace
Denkou Choujin Gridman
Angered by water pollution, Takeshi Todo had Terragaia go into a city water plant to turn the water supply into hydrochloric acid. With its gas, Terragaia could melt circuit panels and burrow underground in the computer world. While Yuka Inoue was analyzing a sample of the tainted water, Gridman alerted the gang about Terragaia. However, Gridman walked into a trap when he entered the plant's computers. His ankles were shackled and was being asphyxiated by Terragaia's gas. Luckily, Ippei had finished designing Thunder Jet and Twin Driller; using the new mecha, Twin Driller broke the shackles and Thunder Jet used the Thunder Ring to muzzle Terragaia. Unable to spray its gas, Terragaia was destroyed by Gridman's Grid Beam.
- Terragaia's body design was later used for the inspiriation for Dazzlba and later Skubone.
- Terragaia might be a tribute to the monster Antlar.
- Terragaia shares his title with Telesdon.
- Powers and Weapons
- Acid Gas: From its mandibles, Terragaia can fire a cloud of toxic acid gas.