User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-75.119.225.228-20121006002039/@comment-3146930-20121006020908

Thanatos was very vague about how the doors work. He basically said they weren't entirely physical and they could only be found by questing for them. From this, we can tell that the doors have some kind of magical properties that we don't know about. Also, the doors act as a way in and out of the Underworld, so there may be other Doors of Death, but the doors in Tartarus and Greece may be the main set, meaning the other doors can't be closed if the on in Tartarus isn't closed, but I'm guessing/