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Built by what is believed to be a Great Being before The Shattering. The aging city has the ability to raise a powerful energy shield and convert it to cloak when need be. It is also believed that the city can engage a hover drive and allow it to float over the surface of the water however, the city is normally held over the ocean floor via a large protosteel connector pipe, called the Plug Lock, forced into the hard rock of the ocean floor.
The city is, itself is defenseless and relies upon the populace to defend it's structure, as the energy shield has been know to deplete the cities' regenerating power supply. This being to case during the many decades it spent underwater, where it was forced to drop its shield and survive only with it's thick outer armor plates. Which have been noted to take a beating from Teridax in Mata Nui form during his takeover if the Matoran Universe.
Mantalis Nui was once an Artifical isand that used to float over the waters, close to the recently formed Arki Nui. The island was a highly advanced Micro city that had to be supported by a large retractable pipe line named the plug lock, that prevented the island from falling into the abyss. The Matoran that thrived on the island where Matoran of healing, when the resident Toa became infected with the 'stigma'.
A disease that slowly corrupts the body with a fatal poison. The Toa was sealed in a cryo chamber and locked in a sleep like state. When the Matorans fell to the Stigma they designed and built several automated drones with a hive mind, naming them the Observers with a task to prevent the stigma from spreading and contain dangerous Toa and Matoran, however in their rush to complete the droids only on profile was added into their system, a profile of the pacifist that were the Matoran inhabitants, meaning all species with violent tendencies were considered dangerous.
After the Observers had departed from the city. Most of the Matoran were placed in the cryo chambers and the few remaining remover the mask from the Toa and placed in inside the chamber of healing. Only after Mata Nui had fallen into his deep slumber as per the production of the stars, the chamber would open and the only non tainted Matoran of healing would wear the mask and become the Toa's replacement and begin the long process of awakening and healing the populace. Then as a final measure to defend the city the last remaining Matoran retracted the plug lock, sending the city plummeting to the ocean floor. The only damage recorded was minor that included the destruction to the exterior per buildings and the flooding of the upper-most levels of the control spire. It is unknown what happened to the final Matoran, however it is theorized he was fully exposed to the 'Stigma' and mutated into something that cannot be predicted.