My thoughts on the spiral (SPOILERS)

I've read a lot of posts and did some research on the manga and it appears to me that the spiral is hidden under the city that was created by an ancient group with cults and etc. Yada yada. I was thinking however what if the Vortex or the spiral is actually a portal to another dimension? Like, what if this portal was preserved or worshiped because it was a looking glass into another realm of existence so this culture built a city over it and worshiped the portal? Now every hundred or thousand of years it re-opens allowing itself to taint anyone near it's grasp. After all what would drive a man to obsession or madness more so than a view of another unexplainable realm?

I'm also aware that the manga explains that the city underground is alive and more or less jealous of no one being able to see it so it makes itself known every thousand years or so. However, this could still work with the vortex/portal idea that this gateway is a living, and breathing entity that commands the gap between this plain of existence and the next. Anyway, even if this isn't the intended case I like to think of the vortex this way. Thoughts anyone?