The Green Belt was supposedly caused by the Kanohi Mask of Life. The entrance to the Chamber of Life is somewhere in the green belt. The Matoran Village in it held another secret: the entrance to the Stone Cord that led to Mahri Nui.
Reidak once pursuited a Le-Matoran down it in the Piraka/Toa Inika Animations, then Thok captured the same Le Matoran before he discovered the chamber that led to the Mask of Life. It was most likely destroyed when Mata Nui rose from the sea.
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