Equantica

Equantica is one of the many underwater cities found in the seas of Equestria, being inhabited by the rarely-occurring fourth pony tribe, the merponies (or hippocampi). It has been one that went missing since Discord's reign causing him to make it disappear into the seas of Destiny Islands. Since it's disappearance, as well as the disappearance of it's most precious protectors, The Trident of Benevolence and The Purple Sea Dragon in the hands of the evil Captain Taiklar Blackheart, and the sea dragon's murder in the hooves of the parentally-abused and bitter councilor, Dead Sea, who decided to help lift Discord's curse the way he wished, including manipulating the royal family rulers Queen Oceanious, and specifically her youngest daughter Princess Marenia, who lost her immunity to the curse's effects of maintaining trust with land creatures, including humans, as punishment for almost blowing their cover with a non-Equestrian horse prince named Prince Derek, especially since their kingdom was under the conquest campaign of a rogue creation of Dead Sea, Geogra, who constantly turned humans and other land creatures against the sea folk, making it especially difficult for Marenia to easily trust humans. However, thanks to Fluttershy, a pony who wished to explore a new environment like the sea after accidentally stumbling upon it's wildlife, it lead to her using her kindness and her friends to build bonds with the merponies, starting with Derek and defeating his evil greedy oil-tycoon competitor Mayor Maximillion, whose rule would pose a threat to Destiny Islands sea life with his oil industry. Dead Sea's crime was discovered and he is currently regressed to a baby to be raised better by Oceanious, Geogra was destroyed, Discord's curse was lifted, and the underwater city returned to it's rightful place in Equestria, while Marenia would live happily ever after married to Derek and having a merpony filly named Harmony, who is still a baby. (Picture coming soon. It's gonna use a lot of pasted material from the two canon undersea scenes in MLP, including the recent one at the beginning of the latest episode.)

History
Before Discord's curse, merponies used to be good friends with ponies before, during, and after the Chaos War. However, when the Chaos War ended, the ponies' hatred and the Windigo Ice Age chased the merponies off into far waters to avoid the blizzards. They have rarely been seen by the other three ponies ever since, even after they had changed and founded Equestria. Equantica was built by a small tribe under the leadership of two Artificial Alicorn descendants of deceased Alicorns of the Sea, Oceanious and her husband, King Barrier Reef, as they have formed a slight bond with life under the sea, and built the city to live in close comfort with them.

More coming soon...

Society
Equantica is not the only underwater city of Equestria as there was a few in comics (Coltlantis in Friendship is Magic Issue #24) and a book (Aquastria in Under the Sparkling Sea), as the two exchange inhabitants with one other even today. The entire kingdom was built in a large coral reef teaming with beautiful undersea life as it's builders wished to connect with the beauty of the sea. To this day, that is still apparent. The city covers acres of sea floor, and contains kelp farms, wildlife sanctuaries, reef parks, table corals that serve as decks for sentient sea life, large rock formations that serve as the base of the towers, large towers made of pearlescent metals, kelp is used as decoration, and various others. They also consider bubbles as a good enough decorative feature as waterfalls.

The coral reefs surrounding the entire city is a rich location as it's where most life across Equestria seasonally migrate, making it an ideal location. These migratory paths are also taken by merponies and other sentient beings. The city's coral reefs are surrounded by open empty waters where the only landmarks are rocks and sea grass, and sometimes kelp forests.

The city has a council which is basically the Unicorn Council of Equantica, as the city has a limited monarchy where, while Oceanious and King Barrier Reef would have the most control, the council would act as a 'Congress' that supports people and keeps the rulers in fair position in their eyes. The royal ruling family has unlimited command of the royal guard, and the council has 75% control over the judicial branch.

Equantica is not just populated by merponies. The occasional ichthyocentaur (centaur with upper human body, a horse's front body, and a fish tail), kelpie, sea monkeys, sirens, nymphs, and sea dragons. Other sea creatures from the books and comics are native here. Other pony tribes, without common physical and/or magical support, are unable to survive here not just because of the risk of drowning, as it is over 100ft deep, the pressure of which can have fatal consequences.