User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130312053730/@comment-3146930-20130315002615

The word Portal is just the easiest way to describe it, as each entrance to the Underworld seems to take the user to about the same area. For example, in The Lightning Thief, Percy, Annabeth, and Grover go all the way to LA in order to gain entrance into the Underworld. When they finally make it, they go down and walk to Hades' Palace. Now in The Last Olympian, Nico leads Percy into the Underworld and they once again walk to Hades' Palace. If the Underworld is really so big that it can extend from one end of the country to the other, then how is it possible that Percy could walk to the palace when entering from opposite ends of the country?

And yes, Annabeth does admit that there could be entrances to the Underworld in the Labyrinth leading all the way down to the Underworld, but think about the Labyrinth. It is a place where space and time don't matter. You could walk a day and go a foot, or walk a foot and end up in Colorado when you started in Florida. I mentioned the 4th dimension as more of a joke (because as we all know, the 4th dimension is time, not space), but what I was saying is that much like Olympus, the Underworld also exists outside of the regular world and doesn't follow the rules of physics. Each entrance into or out of the Underworld is like a gateway (ex. portal) to another dimension.

Also consider that there is an elevator that goes from the Empire State Building to Olympus, which hovers above it. Without the use of some kind of magical teleportation (ex. a portal), then there would be an elevator shaft sticking out of the top of the Empire State Building, which there isn't.

While I admit that I tend to "make up a while bunch of rules" as you put it, the theories I invent explain things that Riordan hasn't explained yet and are based on the reality of the world the series takes place in. It is a world where gods exist and have vast reality warping powers, where monsters roam the countryside, and the world is constantly being saved by teenagers.