"Up ahead lay the Great Barrier, a cliff so high it vanished into the sky and so wide it encompassed the entire horizon."
— Narrator, Legends of Metru Nui
The Great Barrier was the wall of the dome that surrounded the island city of Metru Nui.
The Great Barrier separated Metru Nui from the rest of the Matoran Universe with only six  gates open to access the city. During Makuta Teridax's struggle for power in Metru Nui, he disguised himself as the city's leader, Turaga Dume, and sent most of the members of the Toa Mangai to shut the gates. Unknown to the Toa Mangai, the Dark Hunter Eliminator, hired by Teridax, was stationed at the Barrier to kill the Toa.
The Great Barrier was damaged during the Great Cataclysm, causing several holes and tunnels to open up in the wall. While fleeing the damaged City of Legends with six Matoran Spheres, the Toa Metru fought and imprisoned Teridax here, and then later came back to save the rest of the sleeping Matoran from the Visorak hordes to take them through a tunnel to the island of Mata Nui above.
The Great Barrier, with much of the rest of the Matoran Universe, was heavily damaged following its impact with the planet Aqua Magna during Mata Nui and Teridax's battle. Later, the Great Barrier, along with all other locations in the Matoran Universe, was dismantled.
The barrier has many cliffs along its face, and a large door that connects Mangaia, Teridax's lair, to a ledge in the barrier that looks out over Metru Nui's Silver Sea. There are also several Sea Gates in the wall of the Great Barrier which, along with many underwater Chutes, go through the base of the barrier, leading to other islands.
- Chat with Greg Farshtey, 18 August 2014
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