Iconia
Iconia is the home planet of the extinct Iconian civilization, located in what is now the Romulan Neutral Zone. The planet was devastated by a massive orbital bombardment approximately 200,000 years ago. The Iconians were believed to have been annihilated, although a part of the population may have escaped through their gateways.
The precise location of Iconia was lost to history until 2365, when an artifact bearing an ancient starmap was discovered in an archaeological dig on Denius III. From there, Captain Donald Varley of the Federation starship USS Yamato was able to locate Iconia, and chose to violate the Neutral Zone to reach the planet so as to prevent Iconian technology from falling into Romulan hands. After the Yamato was destroyed by an Iconian software transmission, the USS Enterprise-D and the Romulan warbird IRW Haakona both traveled to Iconia, where Captain Jean-Luc Picard had the last operational structure on the surface destroyed. (TNG: "Contagion")
In 2372, Worf told the crew of the USS Defiant of the events that had taken place on Iconia. (DS9: "To the Death")
Later in 2407, Members of the crew from the USS Boudicca were trapped on the planet Iradon which was trapped in an Interdimensional Rift that caused it to shift universes every 1,000 years.
This proved instrumental in developing a better understanding of the Iconians as Iradonian culture was a direct descendant of the Iconian culture. The Iconian language thrived there and several members of the Boudicca Away team managed to learn this language.
Currently Arania McLaren, a native Iradonian is the only living descendant of the Iconian culture and civilization, although a significantly technologically reduced culture known to exist in the Federation, and more specifically in this universe.
