User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3146930-20121014230006/@comment-4640327-20121021185728

The doors are always changing places. Since they're Thanatos personal door, he cobtroled them and could open and close the DoD as he wished. Nevertheless at some point he was tricked, or made (probably when he was caught) to create them in Tartarus. They exist at multiple places at once and u can only know for certain where they're if u make a quest. A pretty strange analogy can be made with Schrodinger's cat. Also, and u must remember this cause it's absolutely crucial, the underworld isn't a physical place. Once u enter the passages for it, things start getting odd and things get worse as u aproach Tartarus. Tartarus can be everywhere under our feet, or nowhere. To get in or out u got to rely on the fates or the ancient gods. Either way, it's complicated,