One of the main cities that exist in the Kingdom of Atlantis from DC Comics. Traditionally it is considered the capital of the city-state of Atlantis itself, so normally speaking of Poseidonis is also talking about Atlantis, both being used as a simile.
Traditionally, the city is considered to be in the geographical center of the continent and Kingdom of Atlantis, it is also close to the city of Tritonis, both being connected by a series of underground tunnels. Aquaman's Aquacave is also known to be located on the outskirts of this city, serving as the hero's main base of operations.
The name of the city arises as a tribute to the greek god Poseidon, lord of the seas and oceans by the inhabitants of the city.