User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3146930-20121014230006/@comment-3146930-20121021193834

I'm still thinking that there are just multiple doors. I mean people from the Fields of Punishment seem to be getting out just fine (Phineas even said Gaea showed him how to get back faster). At the same time, monsters seem to be able to escape the instant they are sent down to Tartarus. I think that what Thanatos meant when he said find it through questing was that he didn't know where Gaea moved the main Door of Death, the one that needed to be closed so all the other doors could close as a result. So like you can't close Doors B, C, and D if Door A isn't closed. Thanatos was then captured and during that time, Gaea captured his main Door of Death and took it to the Underworld. After he was free, he could only sense that the answer was in Rome and sent them there. After that, he protected the Doors of Death that weren't captured by the enemies and stopped the return of monsters and souls...possibly.

As far as the Underworld (and by extension, Tartarus) not being a physical place, I somewhat have to disagree. I mean in the series it is explained that there are multiple entrances and exits to the Underworld, which in my opinion is the important point. While the Underworld may exist in an alternate dimension like Olympus, if you find an entrance you can enter the Underworld (which explains how Percy could end up close to the same place in the Underworld despite entering in California in TLT and New York in TLO). Tartarus is a physical place in the Underworld, as Percy found a big pit leading to it in TLT, as well as Typhon being sent there in TLO and Arachne building her lair over a passage to it. So just like the Underworld there are ways into it, but unlike the Underworld it isn't designed with a way out.