Atlantis

Atlantis is the location of Poseidon's underwater palace. It is an underwater city inhabited by merpeople and their allies (Cyclopes and monsters who help them). The human myth is that, a Greek philosopher named Plato had discovered the island of Atlantis as a beautiful society which prospered around 9600 BCE. But after its people tried to invade Athens, the island sank into the ocean, never to be seen again. Dozens of possible locations for the city have been suggested, including, Santorini, Greece, the Azores in the Atlantic, and off the coast of Cornwall in New England.

The Last Olympian
Tyson found Percy drifting underwater after Percy escapes from Kronos' grasps and Luke's cruise ship, Princess Andromeda, exploded and took him to the palace. He said the palace was the size of Mount Olympus. There he saw several of mermen in the courtyard. The palace was in the process of being destroyed by the Titan Oceanus whom Poseidon was fighting his own war aginest. In the throne room he met his father Poseidon, his stepmother Amphitrite, his half brother Triton and Delphin, the god of dolphins. Poseidon's appearance is an old man and he is watching a mosaic that mirrors his kingdom, with every building being destroyed in reality is shown on the mosaic. He then sees Oceanus approaching and bids goodbye to Percy so he could fight Oceanus in his true godly form.