On the parallel-universe of Earth-One, Poseidonis was named for the Greek god, Poseidon. It was the capital city of the continent known as Atlantis. Geographically, it was at the center of the continent, near to the neighboring city of Tritonis. Both cities are connected via a network of subterranean tunnels. The super-hero Aquaman maintained his base of operations in Poseidonis during the years that he served as the continent's undisputed monarch. His secondary headquarters though, the Aquacave, was actually located outside the city.
In the years prior to Atlantis sinking beneath the sea, Poseidonis was under the reign of the benevolent king Orin (for whom Aquaman was named).
It is now known as the domed-city of Atlantis and home to Aquaman, founding member of the Justice League.
- Poseidonis first appearance is unknown.[?]