Poseidonis. Atlantis is an underwater kingdom, and the home of the Atlanteans. King Orin is the current king of Atlantis. It is a constitutional monarchy.
Atlantis is the size of a small continent; the capital city is  Poseidonis. Other city-states are  Shayeris, Tritonis, Nanauve, Neptunos, Lemuria and Crastinus. It is not currently part of the United Nations. 
The continent Atlantis sank beneath the surface millennia ago. The Atlanteans used science and sorcery to adapt to life on the ocean floor.
At a time when Orin was absent on
Justice League business, Black Manta attacked the capital. Several explosions were set off in the capital to act as a distraction while tried to steal Starro. He was stopped by Garth and Aqualad, so instead opted to blow it up.
Racial tensions rose, and the
purists attacked "impure" Atlanteans. Topo was one such victim.
Aqualad introduced Superboy and Miss Martian to his parents, Sha'lain'a and Calvin Durham. They were quite pleased to see him again, and welcomed his friends.
Poseidonis September 6, 08:06 UTC-2
After meeting with
Queen Mera and Prince Orm, Aqualad visited the Conservatory of Sorcery to see his friends.
Poseidonis September 6, 19:27 UTC-2
While talking with his old friends
Garth and Tula, Aqualad learned about the purists. They had to break up an argument between conservatory students about that very subject.
Poseidonis September 6, 23:45 UTC-2
Ocean-Master and the purists kidnapped Mera from her royal quarters. She was brought to the S'atiroman Cave and held hostage.
Orm and Aqualad organized a counterattack to rescue Mera. Orm deliberately sent Kaldur to an ambush at the
Old Roman's Trench.
Poseidonis September 7, 07:01 UTC-2
Ocean-Master, Orin thanked his protégé for the safe return of his queen and unborn son.
Poseidonis June 13 23:45, UTC-2 Time Zone
Miss Martian traveled to Atlantis's capital to meet up with Lagoon Boy, who had been living in the city to recuperate from wounds sustained three weeks earlier. They left in the Bio-Ship together.
Weisman, Greg (2012-03-23). Question #14540. Ask Greg. Retrieved 2012-03-24. ↑
Hopps, Kevin (writer) & Oliva, Jay (director) (March 4, 2011). " Downtime". . Young Justice Season 1. Episode 8. Cartoon Network. ↑
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Weisman, Greg, Kevin Hopps (w). Jones, Christopher (a). Atkinson, Zac (col). Sienty, Dezi (let). Chadwick, Jim (ed). " Under the Surface..." Young Justice 14 (March 21, 2012), New York, NY: DC Comics
Hopps, Kevin (writer) & Oliva, Jay, Divar, Tim (directors) (April 14, 2012). " Usual Suspects". . Young Justice Season 1. Episode 25. Cartoon Network. ↑
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Weisman, Greg, Kevin Hopps (w). Jones, Christopher (a). Atkinson, Zac (col). Sienty, Dezi (let). Chadwick, Jim (ed). " ...Here There be Monsters" Young Justice 15 (April 18, 2012), New York, NY: DC Comics ↑
David, Peter (writer) & Zwyer, Mel (director) (March 2, 2013). " Intervention". . Young Justice Season 2. Episode 18. Cartoon Network.