Poseidonis, named for the Greek God, Poseidon, was the capital city of the continent known as Atlantis. Geographically, it lies within the center of the continent, near to the neighboring city of Tritonis. Both cities are connected via a network of subterannean 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 leading up to the Great Deluge, Poseidonis was under the reign of the benevolent king Orin (for whom Aquaman was named). Poseidonis was often the target of vicious raids from nomadic tribes of savages, but the scientists of Poseidonis discovered that they could protect themselves by constructing a dome around the city made of pumice. The specially treated pumice rock was rendered translucent and was porous enough to allow air and moisture in, but was sturdy enough to fend off attackers. Though most Poseidonians praised Orin for his ingenuity, some, such as his brother Shalako, felt that the construction of the dome was an affront to Suula - Goddess of the Sky. Shalako warned that if they persisted in cutting themselves off from Suula's grace, then they would ultimately suffer her wrath.
Shalako's predictions did in fact come to pass, though whether Suula was directly involved in the coming catastrophe is subject to interpretation. A giant meteor slammed into the planet Earth, causing massive geological upheaval. Tectonic plates shifted and the whole of Atlantis sank beneath the waves. This event came to be known as the Great Deluge (sometimes referred to as the Cataclysm). Poseidonis survived the flood, owing largely to King Orin's dome. Those of neighboring Atlantean cities however, were not so fortunate, and millions were killed. As the city settled on the ocean floor, it came to rest atop the remains of the very meteor that destroyed them. 
For several years, the surviving Poseidonians were forced to remain in the restricting confines of the city. During this time, Shalako had amassed a religious following who condemned the technocratic methods of the other Poseidonians, and he mounted a mass exodus from the city via the underground tunnels. Using Shalako's magic to protect themselves from the crushing waters, these Shalakites, migrated to the ruins of Tritonis and established a new life for themselves.
Three years later, the Atlantean scientist Thorne, developed a serum that could alter an Atlantean's physiology, giving them amphibious characteristics. In truth, these scientists had been experimenting on this type of work for several years preceding the great flood. The project was kept secret for a variety of reasons and King Orin insisted on being the first to sample the serum. The serum worked, and Orin was able to leave the domed city and swim freely in the ocean. His body adapted to the changing pressures and he was able to breathe water as easily as any normal human could breathe air. Atlanteans now had the means to leave the city. King Orin offered this miracle serum to the Shakalites of Tritonis, but his brother refused. Many of his followers however, accepted Orin's gift and were granted the ability to live underwater. Angered at what he believed was Orin's surreptitious attempt to unseat him, Shalako cursed his former followers, transforming them into a race of scaly Mer-people. 
Points of Interest
- AHARD-1 was a mental health facility and part of the Atlantis Health and Rehabilitation Department. When Mera suffered a nervous breakdown following the death of her son, she was remanded to the care of AHARD-1. During the jellyfish invasion, AHARD-1 was turned into a triage unit.
- This was the central correctional facility of Atlantis. It was mostly used for imprisoning political prisoners.
- Atlantean Royal Palace
- The royal palace of all Atlantis was located in the heart of Poseidonis and had been the seat of power for all of Atlantis' rulers for millennia.
- Salt Processing Plant
- The Salt Processing Plant was used to pull salt from sea water and refine it for commercial use. Salt was Atlantis' number one export. Some of the salt refineries were destroyed when a race of giant jellyfish laid siege to Atlantis.