The Nui Caves were a network of caverns located beneath the surface of Voya Nui. Axonn led the Matoran down here to keep them safe from harm before Voya Nui returned to its native continent. Nektann could also be found here.1
Notes
The Nui Caves were a network of caves on Voya Nui rumored to extend far beneath the island.2
The Nui Caves were actually an upper part of the Av-Matoran homeland.3