The Holon Ruins are the ruins of a civilization that existed during the same time as 15th Century Ransei, and go deep underground.
An area that was used by Aura Guardians a long time ago for training purposes containing certain inscriptions and guarded at the end by a number of Metagross.
A section of the ruins that, due to some form of weird space shenanigans that likely frustrated Palkia to no end, links up to Mt. Ohmeg. The catacombs have been broken down and filled with magma. There seems to be some form of link to the Spirit World, as there are pieces of the ruins that are invisible to mortals unless a Ghost solidifies them. Deep within the Catacombs lies the shrine of Keetongu, a Delta Regigigas that has a place in Insula Nui legends.