Codrex

The Codrex was a massive sphere located in the swamp of Karda Nui. It was where the Toa Mata entered their canisters to await the day they were needed, and was where they had to go to reawaken the Great Spirit Mata Nui. The Codrex was protected by an energy field that not even the power of a Toa or a Makuta could penetrate. The only way to enter was to place a Keystone in a special niche in the field.