Karda Nui

"It’s a big cave. With a swamp in it."

- Mutran

Karda Nui (also known as the Universe Core or The World that Feeds the World) was a mysterious world where Av-Matoran resided. It's name meant "great heart" in Matoran language.

History
Karda Nui was colonized by the first type of Matoran, Av-Matoran. During the Av-Matoran's colonization of Karda Nui, the Toa Mata were assigned to guard the region. About 100,000 years ago, the Av-Matoran left the core because their work there was done. After the Toa Mata were chosen to be Great Spirit Mata Nui's personal guardians, they also left the core. When Voya Nui rocketed upwards from the mainland, it ripped a hole in both the mainland and Karda Nui. However, instead of flooding the mainland, the water came through as a waterfall and thus all the water went into Karda Nui instead, creating the Swamp of Secrets. Sometime during this, the Av-Matoran returned to Karda-Nui, though the reason why is unknown.

The hole in the dome of the core was sealed when Voya Nui returned to it's former position. The Ignika was used here by Toa Mahri Matoro, reviving Mata Nui from death by sacrificing himself.

Later, a number of Makuta invaded the core. The Order of Mata Nui was relying on the Toa Nuva to prevent the invasion, and would provide them with any available help. Due to the Makuta invasion on the Universe Core, the Ignika started the countdown to drain all life from everything in the universe. Just before The Ignika awoke Mata Nui, the residing Matoran were evacuated. Karda Nui was then incinerated by the Energy Storm.

Location
Karda Nui was located right underneath the Mainland and was protected by powerful shields, which also prevented the Staff of Artakha from repairing it. It was around the same size as the mainland, and it was located within it's own dome. The Universe Core was located a bit deeper into the planet than the rest of the Matoran universe. Karda Nui's barriers were strong enough to hold back the power of the Staff of Artakha and stop the Toa Nuva from accessing their Toa Suva, thus preventing them from switching masks.

Trivia

 * Karda Nui was made by the great beings to "circulate" energy for the universe and Mata Nui.
 * The "Karda" from Karda Nui is derived from the english word "cardiac" (cardiac means heart).
 * Karda Nui was literally Mata Nui's heart.