Great Barrier

The Great Barrier was an incredibly huge wall of rock and solid Protodermis that bordered Metru Nui and the entire Silver Sea.

Trivia

 * The Great Barrier is really the undergrond dome that housed all Metru Nui.
 * The "ceiling" of the dome had two "suns" that were actually holes in Mata Nui´s surface and "stars" (they´re obviously not normal stars). After the Great Cataclysm one hole was covered up, but later uncovered by the power of the Staff of Artahka.
 * It was also the place were the Toa Metru fought Teridax, in the Silver Sea.
 * There was a hole in it that resulted to be a tunnel that lead to the planet´s surface.