Aqua Magna

Aqua Magna, also known as the Endless Ocean Planet or the Great Sea, was the planet which Mata Nui crash-landed on as a result of the Great Cataclysm and a moon of Bara Magna.

History
Aqua Magna was originally the ocean of Spherus Magna. When the Shattering occurred, Aqua Magna broke loose and became a moon of Bara Magna.

When the Great Spirit, Mata Nui, completed his mission, he was returning to Bara Magna when he fell into a coma because of a virus which the Brotherhood of Makuta had infected him with, and "crashed" on Aqua Magna. In the impact, part of the Mainland inside the Matoran Universe broke loose, punched out of Mata Nui's chest and subsequently rose up onto the surface of Aqua Magna, forming an island which was later known as Voya Nui. After crash-landing, many islands of the Matoran Universe rose to the surface. Mata Nui's camouflage system also formed an island over his head, an island which was later known as the island of Mata Nui.

After the Toa Mahri destroyed the Stone Cord, Voya Nui sank back to its original position, destroying the area known as Mahri Nui in the process.

When the Toa Nuva awoke the Great Spirit, the Matoran Universe was reformed. Makuta Teridax located and possessed Mata Nui's "brain", a bank of machinery far beneath the surface of Metru Nui. When Teridax arose as the new Great Spirit on Aqua Magna, the Island of Mata Nui broke apart, destroying the landmass on the surface of the planet.

As Mata Nui, now controlling a prototype of his body, began to reform Spherus Magna, Makuta left for Bara Magna to kill the great spirit. During their battle on that planet, Teridax threatened to destroy Bara Magna with a blast of gravity power. However, Mata Nui redirected this blast into space, striking Aqua Magna and Bota Magna. This blast caused them to fly back toward Bara Magna. Right before the moon rejoined with Bara Magna, Mata Nui pushed Teridax, causing a chunk of Aqua Magna to smash the Great Spirit Robot's head, killing The Makuta once and for all. Aqua Magna and Bota Magna then fell back into their respective locations on Bara Magna.

Trivia

 * "Aqua Magna" is Latin for "great water", or "great sea".
 * Aqua Magna's apperance is similar to that of Earth as the majority of both planets is liquid.