The Great Barrier was a cliff so high that it vanished into the sky and so wide that it encompassed the entire horizon. It was located at the edge of the silver sea around Metru Nui.1
Notes
The Great Barrier was a large rock wall completely encircling the silver sea. It had openings that allowed beings to sail to other islands, but these openings could be closed, preventing travel.2
The Great Barrier was the wall of rock around Metru Nui, not necessarily its ceiling.3