Metru Nui

Metru Nui, the City of Legends, was the subterranean island where the Matoran evolved and inhabited until Makuta took over. It was found beneath Mata Nui, in the middle of the Silver Sea. Metru Nui and its troubles is the central plot of the Bionicle.

History
The city of Metru Nui was founded and built roughly a hundred thousand years ago, around the same time that the Brotherhood of Makuta and Order of Mata Nui were founded. Metru Nui's foundations were laid by the Great Beings, and the rest was built by Matoran. It is also known that the First Toa was somehow involved in this. Since then, it has been the ecumenopolis homeland of the Matoran for millenia.

Around eighty thousand years ago, there was a civil war called the "Great Disruption." Little is known about the four-hundred-year conflict, but the Brotherhood of Makuta ended the fighting and banished the war leaders to a place known as "The Pit". The Order of Mata Nui placed an agent in the city to ward off further conflicts.

Metru Nui was definitely not devoid of conflict, however. 1000 years ago the Brotherhood of Makuta was already long corrupted and had devised a plan to attack Mata Nui and take over the Matoran Universe. The Makuta of Metru Nui disguised himself as Turaga Dume and hired three Dark Hunters, Nidhiki and Krekka inside the city and "Eliminator" outside it, to get rid of the Toa Team protecting Metru Nui. He succeded, but not before new Toa were created to guard the city: the Toa Metru. For their first mission, these new Toa destroyed the Morbuzahk plant that was threatening the city. Makuta then finally revealed himself and attacked Mata Nui, starting the Great Cataclysm in which Metru Nui was shaked by an earthquake. After that he started gathering all the Matoran and putting them into Matoran Pods. The Toa Metru then faced him and imprisoned him in a protodermic cage. However, the Visorak hordes soon took over Metru Nui but were defeated by the Toa Hordika with the help of Keetongu and the Rahaga. In the process, the Toa accidentally released Makuta from his prison, but managed to save the Matoran and bring them all to Mata Nui. Metru Nui was then watched over by the real Dume, Keetongu and the Rahaga for 1000 years. During that time only one gate to the waters around Metru Nui was open.

Metru Nui would wait until Makuta was defeated again by the Toa Mata, and later, Takanuva. After the battle over the Mask of Light, the gate to Metru Nui via Mangaia was once again opened. The Matoran made their way to their "old" home, and settled it once again. The battle against Makuta was taken to other fronts, and Metru Nui was finally safe. The Matoran first settled in Po-Metru because it was the least damaged. They started to rebuild Metru Nui but weren't finnished before the Staff of Artakha was used on Daxia and repaired the whole island. It was recently almost destroyed by Toa Jaller who was about to unleash a Nova Blast somewhere in the Endless Ocean, but was teleported to Metru Nui and had to calm himself to not destroy the city.

Environment
The city of Metru Nui was actually underground, far below the island that came to be known as Mata Nui. The Matoran living there were generally unaware of this, however, mists obscured the cavern walls (called "the Great Barrier") and gave the appearance of a normal sky. There were also two holes in the ceiling, which looked like two suns to Matoran living on Metru Nui. These two holes had a barrier over them so water or other things would not be able to fall through. One hole is believed to be in Naho Bay, while the other under the ice of Mount Ihu. The city also stood in the center of a vast "Silver Sea" of Liquid Protodermis.

District System
The population of Metru Nui was dispersed throughout six "districts", each specializing in a specific element and occupation, and the central point where many things took place.

Coliseum
The Coliseum was at the center of the city of Metru Nui, where the borders of the six districts met. It was where sporting events such as Akilini took place, as well as being the main power station for the city. It also served as the Turaga's home. When the Visorak invaded the city, they made the Coliseum their main nesting grounds.

Ta-Metru
Ta-Metru was the place where Protodermis was molded and manufactured after it was rid of its impurities in Ga-Metru. Because of all the smoke and heat from the foundries and forges, Matoran from other districts did not like to stay here long.

Ga-Metru
Ga-Metru was the center of learning in the city, where the schools and research labs were located. This was also the district where Liquid Protodermis was taken from the Silver Sea and purified so that it could be used elsewhere, there were canals for Liquid Protodermis everywhere. Ga-Metru was very peaceful. The Great Temple was also located in Ga-metru.

Le-Metru
Le-Metru was the transport hub of the city. Le-Matoran were in charge of the entire city's transport network, particularly the system of chutes that moved Matoran and things between Metru. The flurry of activity and the number of wires strung about gave the district the feeling of an urban jungle and of barely controlled chaos. Le-Metru's Moto Hub was the largest chute station in Metru Nui, as well as the administration building for chutes, where the main controls were located. There was also a Test Track, where new vehicle designs were made and tested by volunteer Matoran.

Onu-Metru
Onu-Metru was home to the Archives, an underground museum complex where Onu-Matoran archivists worked hard to preserve artifacts of Metru Nui's history. The archives have actually expanded again and again, to the point where they stretched underneath the whole city. Besides just artifacts, Rahi were also kept in the Archives.

Po-Metru
Po-Metru was the most natural of all the city districts, as it was made up mainly of rocky mountains and canyons. Po-Matoran liked to work outside, assembling goods and carving statues. Rahi attacks and stampedes were somewhat common in this district. Po-Metru's claim to fame were the Sculpture Fields, a large expanse where Po-Matoran worked to carve giant statues much too large for any building.

Ko-Metru
Ko-Metru was known best for its Knowledge Towers, which are crystalline structures that could reach so high that their rooftops were often covered with ice. Ko-Matoran were scholars and seers who studied and analyzed, trying to predict the future. The district was kept incredibly cold, in order to better preserve written documents. Ko-Metru was the quietest district in the city, as Ko-Matoran hated to have their work interrupted. The mythical Rahi Keetongu also made his home in a Ko-Metru cave.

Matoran
There were six types of Matoran in Metru Nui: This was because these six types were considered by the Great Beings as the most important types of Matoran. Recently, the Matoran of Karzahni have come to Metru Nui with at least one Av-Matoran. It is possible that there are more Matoran of other elements.
 * Ta-Matoran
 * Ga-Matoran
 * Le-Matoran
 * Po-Matoran
 * Onu-Matoran
 * Ko-Matoran

Transportation
The beings of Metru Nui moved throughout the city using "chutes", air-filled magnetically-powered tubes in a network managed by Le-Metru. However, the chutes were not the only way to travel around Metru Nui. There were also Airships, Moto-Sleds and many other vehicles Matoran used for transport. Cargo was transported with Cargo Haulers and Vahki Transports were used by Vahki to transport Rahi or other materials.

Daily Life
Most Matoran stayed in their districts, working in a specific job type. The Ta-Matoran worked using the molten protodermis to create Kanohi masks, Kanoka disks, and other tools. The Le-Matoran were in charge of the chute system, the Ga-Matoran specialized in education, and the Po-Matoran were good in architecture, sculpting, and craftsmanship. Onu-Metru ran mining and the Great Archives, a massive zoological, scientific, and historical project. Ko-Matoran would meditate in their knowledge towers, providing needed insight and philosophy. Metru Nui would run this way for a long time, into the days of its destruction.

Crime
In a city as large as Metru Nui, unsavory folk were commonplace, and so were their deeds. Everything had value, stolen or not. This was exploited. Metru Nui had a vast black market, with many customers and products, ranging from counterfeit widgets to a wild rahi. These are the main crimes:
 * Forgery
 * Disk Dueling
 * Smuggling and Theft

Vahki
The Matoran were protected and policed (sometimes violently) by mechanical inventions like the Vahki. This force held powers of coercion and powerful persuasion to help them deal with criminals. The Vahki that were not destroyed in the Great Cataclysm were slaughtered by the Visorak hordes. It is extremely unlikely that any Vahki exist.

Toa
Metru Nui has been protected by Toa Teams throughout the ages, whether slaying legendary Rahi or serving and defeating the Brotherhood of Makuta. Toa passed power down to ordinary Matoran, continuing the cycle of having venerable superheroes to defend the city. Some of the teams protecting Metru Nui were:
 * Lhikan's Team
 * Toa Metru\Toa Hordika
 * Toa Mata\Toa Nuva
 * Takanuva
 * Toa Mahri

Disks
Kanoka disks were used as common weaponry for the Matoran. These disks had adaptive powers according to their codes and elements. Unfortunately, this developed into the popular but illegal sport of Disk Dueling.

Behind the sets

 * Fans have translated "Metru Nui" into "Great City". This means that Ta-Metru means fire city, Ga-Metru water city, and so forth.
 * Metru Nui has appeared in most Bionicle sources, since Mata Nui and its inhabitants are actually part of Metru Nui.