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Atlantica is the kingdom is located beneath the Atlantic Ocean featured in The Little Mermaid franchise. The kingdom is ruled by the sea king, King Triton and the sea queen, Queen Athena until her death. Atlantica's most famous residents are none other than Triton's free-spirited and adventurous daughter Ariel, her best friend Flounder, and the court composer crab Sebastian. These friends usually get Atlantica in hot water but clean up the mess they sometimes start.
Triton's Palace: The home of King Triton, Queen Athena, Ariel's sisters, and Sebastian. The palace holds bedrooms for every member, a large dining room, many hallways, a concert hall, and a treasury.
The City: The area where the actual residents live in Atlantica. The city surrounds Triton's palace.
Ariel's Grotto: Ariel's current secret room, hidden in a cave. A large boulder hides the entrance. Ariel's grotto was once filled with many human items such as paintings, weapons, forks, and spoons. It has held two separate collections of human items, all gathered by Ariel. Ariel was forced to destroy the first collection herself, after Ursula enchanted the objects to attack Atlantica. Ariel restarted her collection, but most of the items in the grotto were destroyed by King Triton in the film. However, at least twelve years later, the grotto still held Ariel's collection, either as a writers' oversight or implication that Triton repaired/replaced the items in the collection after the events of the original film.
Ursula's Lair: Around the outskirts of Atlantica. The home of Ursula, and her eel henchmen Flotsam and Jetsam. Here Ursula cast spells on merfolk and perform great pieces of magic. The lair itself is actually the remains of a giant deceased creature. The entrance houses many polyps, all who were previously merpeople whom had not been able to uphold their bargains with Ursula. The lair still stands today but is abandoned.
Ship Graveyard: A place filled with small sunken ships. Ariel and Flounder visit the graveyard often to search for human things.
Marketplace: Area where most people shop. In the animated series, Ariel visits here often with Urchin and Flounder.
Morgana's Lair: An area above and below where Morgana, Undertow, Cloak, and Dagger once lived. Here is where Morgana would also cast magic spells to try to turn Undertow back from a piranha to a tiger shark. Morgana has also stolen Ursula's Magic from Ursula's lair to turn Melody into a mermaid. However, Morgana has not used enough of Ursula's magic on her so she could remained a mermaid. The lair has caved in after Morgana's defeat by King Triton freezing her in an eternal thick layer of ice. Undertow, Cloak, and Dagger's whereabouts are unknown.
Kingdom Hearts Series
Atlantica is a world in the games, encompassing not only the sea but also the land where Prince Eric resides. In this world, visitors are given magical forms to survive and adapt, such as the case with Sora, Donald, and Goofy, who were given aquatic forms to swim underwater. The world seems to also hold lore on the Keyblade and its history, as King Triton was able to recognize the weapon and the danger it possessed. Atlantica appears in Kingdom Hearts, Kingdom Hearts: Chain of Memories (though as in illusion in this game), and Kingdom Hearts II, all of them following a plot similar to the original film, though in Kingdom Hearts II, events there play out almost similar to what happened in the original film.
Atlantica is most likely based on the mythological city Atlantis (as implied by the name).
Although the kingdom is known as Atlantica, it was never referred to as such in the original film. It was not until a Mouseworks novelization was released in 1993 that the term was coined. The TV series was the first animated medium to utilize that name. The name would stick in the sequel and other Disney media afterwards.
In large part because of this, the Disney Comics serial for The Little Mermaid, most notably the story arc Serpent-Teen, utilized the name Mermaid City.
Using images from the prequel Ariel's Beginning and the TV series, it can be assumed that mermaids do not receive their sea-shell bikini tops until they are teenagers (as the oldest of Ariel's sisters are pre-teens and are repeatedly seen wearing bandeau-like breastbands as toddlers).
It is theorized to be within range of the coast of Denmark (Hans Christian Andersen appeared in an episode of The Little Mermaid Animated series, and Return to the Sea's map indicated that it was near Greenland), as well as possibly Malta or the Northern coast of Africa (the architecture of Eric's kingdom resembles that of Malta (which has heavy influence from Italy and France), and the palm trees and weather imply a Southern Mediterranean climate). Still, it is also accessible to the entire Atlantic Ocean, as it is revealed that Sebastian was born off the coast of Jamaica (hence the accent) in another episode of The Little Mermaid animated series. This could lead to it being somewhere near Gibraltar or else the coast of Morocco or Algeria.