The Kini-Nui was the Great Temple on the island of Mata Nui.
The Kini-Nui, the main temple, was in the exact center of the island of Mata Nui on the far side of Mount Ihu from Po-Koro. It was a stone temple located in a large clearing to one side of the slopes of Mount Ihu in the center of the island.
There was a temple pond within sight of the Amaja Circle. Kini-Nui was also referred to as the Great Temple. It was surrounded by mountains. There was a giant head carved into the stone at the edge of the plateau, covered without countless years' worth of dirt and grime. Kini-Nui was considered a sacred place. Steps led from the upper plateau to the lower plateau where the lower temple was located.
It was referred to as the Great Temple of Kini-Nui.
When fitted together, the Makoki Stones formed a key that opened a passage from Kini-Nui to Makuta's lair.
The Kini-Nui was a large temple dedicated to the Great Spirit Mata Nui. It was long a site for meditation and contemplation. It was partially built by the Great Beings and completed by the Turaga and Matoran after their arrival on Mata Nui.
Kini-Nui was basically a retractable sensor array that Mata Nui used to monitor the surrounding environment when he was on a planet. Had Mata Nui been awakened in the normal way, it would have retracted, but because of the abrupt nature of the awakening, the array was jettisoned instead of being retracted.
The Kini Nui was an integral part of Mata Nui's face, built by the Great Beings, but used more as a "probe" than anything. When Mata Nui fell over, it was extended to rest on the camouflaged layer of dirt and rock above his face. It kept Mata Nui's consciousness informed of surface conditions and provided a safe route for the Matoran and Toa to access his exterior as needed. If the re-start of Mata Nui had been done in proper sequence, the Kini Nui would have been retracted back into its slot on Mata Nui's face. However, since the Toa Nuva had to jump-start his heart with the Ignika, non-essential systems were bypassed, and it was essentially jettisoned.
It might have been possible for more than one Kini-Nui to have been created.