|Cavern of Light|
|Location||Cavern near Ta-Koro|
The Cavern of Light was a massive underground cavern near Ta-Koro. Its walls were studded with many Lightstones and thus was the primary source of the luminous crystals for Onu-Matoran miners. Its entrance was sealed by a stone door, which could only be opened by a key owned by Kaj.
At one point, the access tunnel to the Cavern of Light was flooded by a rogue lava flow from Ta-Wahi; as a result, the Onu-Matoran were unable to access new supplies of lightstones, and thus Onu-Koro and all its surrounding mines were plunged into darkness, becoming reliant on scant torchlight for illumination. The problem was eventually solved by Takua, who used a lavaboard to cross the flow and activate a pump on the other side, draining the magma. After this, the Onu-Matoran were able to unlock the Cavern and resume mining for lightstones, illuminating Onu-Wahi once again.
After the rediscovery of Metru Nui, the island of Mata Nui was abandoned, and all mining ceased in the Cavern of Light. It was presumably destroyed by the Bohrok, unleashed by the Toa Nuva, and later completely obliterated along with the rest of the island when the reawakened Mata Nui rose from the depths of Aqua Magna.