User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130312053730/@comment-3146930-20130312102719

Ya, you have to think Fourth Dimensionally. It isn't like you could drab a shovel and did your way down to the Underworld, but it still exists in a type of dimensional pocket (like Olympus). The Underworld was many ways in and out; it is the reason that Percy could enter the Underworld in LA and Central park in NYC, and still end up in about the same place. Each doorway to the Underworld or even Tartarus is like a wormhole to the Underworld, making distance irrelevant.