Karda Nui

History
Karda Nui was colonized by the first type of matoran, Av-Matoran, or the Matoran of Light. When Karda Nui was first colonized, it was just a huge cavern in which the stalactites on the "ceiling" were beyond the clouds and the only habitable place was on the vast sandy plain that rested on the bottom of Karda Nui. The only other sapient beings in Karda Nui during the time of the matoran colonization were lightening bolt-like beings called Avohkah. The current status of the land and its inhabitants remains unknown as of now. 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 sealed themselves up in the Codrex.That caused a massive and powerful energy storm to suddenly appear in Karda Nui.The avohkah were heralding the storm as they were it's start .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.

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 while killing himself. Recently, a number of Makuta have invaded the core. The Order of Mata Nui is relying on the Toa Nuva to prevent the invasion, and will 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.

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 main land, 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.