Atlantis

Atlantis is the location of the sea god Poseidon's underwater palace. It is an underwater city inhabited by merpeople and their allies.

History
The human myth is that a Greek philosopher named Plato had been told the story of the island of Atlantis by his father, Solon, who had been told the story by an Egyptian priest who was carving it into a column for a temple. The island was in the era 9600 BC. However, after its people tried to invade Athens, in a single night and day of misfortune the island of Atlantis sank into the depths of the ocean. Dozens of possible locations for the city have been suggested, including Santorini, Greece, Azores in the Atlantic, and off the coast of Cornwall in New England.

The Sea of Monsters
While not seen, Tyson got an internship working in the Underwater Forges of the Cyclopes to make weapons for the Olympians for the coming war.

The Titan's Curse
Percy sent Tyson an Iris Message in Atlantis, where Tyson talked about his new job and how some of the older sea spirits were making trouble for Poseidon in Atlantis, waging war against him.

The Battle of the Labyrinth
Tyson gave Briares directions to Atlantis to show the other Cyclopes old ways of forging weapons.

The Last Olympian
Tyson found Percy Jackson drifting underwater after Percy escapes from Kronos' grasps and Luke Castellan'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 against. 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, as he reflects the state of his palace, 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