Southern Continent

The Southern Continent (also known as the Mainland) was an unknown landmass that Voya Nui broke off from. Karzahni sent most of his "fixed" Matoran there when he was done with them. The mainland was underground, on the same level as Metru Nui and in a dome that was far south of it. When Voya Nui split from the continent, a huge hole was punched in its dome, flooding the landmass with water from the Endless Ocean. Makuta Mutran was assigned to the center of the Mainland. Matoran from Voya Nui and Mahri Nui traveled to the Mainland before the The Cord was destroyed.

Out-of-story
Greg Farshtey confirmed that there is a northern continent. Greg also confirmed that Karda Nui is under The Bionicle Mainland.

In the future, the BIONICLE Mainland might have a greater story importance.