The core of the Matoran universe, Karda Nui was one of the most vital spots in all of the Great Beings' creation and the second to be constructed (following Metru Nui). It was a vast cavern of unimaginable size, traditionally barren except for the Codrex. For most of its existence, it was the site of huge energy storms and not habitable.
Matoran called Karda Nui the "world that feeds the world."
Karda Nui was the legendary core of the universe.
There was a western portal which served as an exit out of Karda Nui.
Even when it was just getting started in the center of the swamp, the energy storm in Karda Nui was terrible and beautiful at the same time, light and power unleashed in a fury that nothing could compare to. Even Takanuva, releasing all his power in one mighty blast, could never have equaled it.
When Mazeka skimmed over the surface of the swamp with his swamp strider, occasionally a tentacle or some other part of a creature would emerge from the muck and try to grab the vehicle.
After the quake during the Great Cataclysm, great stalactites broke off from the ceiling of Karda Nui and buried themselves in the ground. Av-Matoran, displaced from their homes by the quake, built sky villages atop the upturned bases of the stalactites. These villages were linked to each other by vine bridges. Transport between villages was also accomplished by means of flight packs worn by the Matoran. After the Makuta invasion, some of the villages were protected by lightvines, luminescent plant life from the swamp below.
When the quake struck, water from outside of the domes universe flooded into Karda Nui, turning the ground into a vast swamp. The murky waters were mutagenic, transforming any living thing that came in contact with them. Strange plant life grew here as well, all of it twisted, some of it dangerous.
The mist that separated the swamp from the sky was impossible to see through. The Matoran only went to the swamp when necessary. The swamp was vast.
Karda Nui was known as the "Heart of the Universe." It was situated directly below the Southern Continent, which formed the roof of Karda Nui's dome.
Karda Nui was bigger than the island of Mata Nui, possibly over two times.
The Codrex and the walls of Karda Nui were made of the same substance.
- Sky villages
- The Swamp