Mahri Nui

Mahri Nui was an underwater colony of Matoran on Aqua Magna.

History
Mahri Nui was once a coastal village on Voya Nui, which was then part of the Southern Continent of the Matoran Universe. This changed when Voya Nui broke off from the Southern Continent and was subsequently ejected from the Matoran Universe, floating upwards to the surface of Aqua Magna. Mahri Nui remained in this position as a part of Voya Nui until Mahri Nui's foundations collapsed and it broke off from the rest of the island. Mahri Nui drifted to the bottom of the ocean, taking all of its Matoran inhabitants with it, but the population survived thanks to the presence of a large amount of harvestable air bubbles that were released when Mahri Nui collided with its new resting place.

One day, a team of Toa arrived on Mahri Nui in search of the Kanohi Ignika, a legendary mask which they hoped would help them save the Matoran Universe from impending doom. The subsequent efforts of these Toa in protecting the Matoran Universe and the Matoran of Mahri Nui prompted the Matoran to name these Toa the "Toa Mahri".

In the Battle of the Halves/Alternates, Blackout, Overlord Blackout, Ynot, and Hollow Ynot were all sucked into a crack in time which transported them to the Mahri Nui of an unknown time period. All four of them were immediately surrounded by aquatic Rahi, so Ynot tried to neutralize them with a blast of chain lightning, but this also knocked everyone unconscious until they were sucked through another crack that took them to Z'traa.