Mahri Nui

Mahri Nui or Lost Land, is the former site of a Matoran village, which sank into the sea centuries ago (though is still connected to the main island). The Matoran living there survived, having domesticated some herd Rahi called Hydruka that harvest air bubbles for breathing. However, the land sank dangerously close to the Pit, where the Order of Mata Nui condemns the world's most vile beings. Some of those beings, a group of warlords called the Barraki, are now terrorizing the Matoran.

After the Mask of Life dove into the ocean, some of the Matoran found it, but it has since been taken from them by the Barraki.