This is not an official term.
This was an inaccessible spot below the Metru Nui Coliseum. Makuta Teridax was only able to reach this place because he could slip through minuscule cracks as antidermis. The only way for someone else to access it was to shatter the foundation of the Coliseum, which would bring the structure down. Teridax came here to execute the final stages of his Plan. According to Norik, all of the secrets and hidden knowledge about the universe and its workings were somewhere beneath Metru Nui, in a place no Toa, Matoran, Turaga, or other intelligent native of the Matoran universe had ever been.
The tunnel leading there was dark and dank. At one point, there was a series of very ancient inscriptions on the wall. The language did not appear to be Matoran; Bomonga was able to make out only a little bit of it, and he hypothesized that it was some sort of root language for Matoran. Bomonga thought it appeared to be a record of something, and it mentioned the name "Bara Magna." [It is not clear if this language was Agori.] At one point, a powerful magnetic force pinned the Toa Hagah to the wall. The tunnel wall proved impervious to their elemental powers. Molten protodermis then flowed toward the trapped Toa.
The Toa Hagah spent days [since they took as look as the Toa Mahri took to get to Artidax?] to get to the magnetic trap. After this, they continued for hours until reaching the chamber.
The tunnel ended in a mid-size chamber lined with sophisticated machinery. In the chamber were two corpses. They appeared to have been dead for many thousands of years. They wore armor and somewhat resembled Toa, but they were clearly radically different. One was in red armor, the other in green. They appeared to wear masks. Miserix determined that their armor was not made of protodermis, and that it was likely that no part of their bodies was made of protodermis, from their organic tissue to their masks. This meant that they were alien to the world as they knew it.
This place was as close to the mind of Mata Nui as it was possible to be. Destroying this place would destroy his mind and doom the Matoran universe.
The inscriptions that the Toa Hagah found beneath Metru Nui were made before Mata Nui left Spherus Magna. Mata Nui was not aware of these inscriptions.