Quite uncertain even after reading 15 beforehand, because everything is being referred to as "this" and "that". My understanding of it is there are three factors - the island, probably meaning the human civilisation on it; the thing Hajime showed Aoi that supports the island; and whoever/whatever is behind the typhoon that wants to destroy the thing. In this case the thing under the island is probably benevolent. The other possibility is the thing is malevolent and summoned the typhoon, as Hajime suggests it has a hand in the war, but there are too many problems with that - why will the ancestors build the island on top of it, why will it destroy itself, and why will the 'island' ride on it to run away from it? So IMO the typhoon is likely a third party and the thing some kind of knowledge preserver.