User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-198.228.228.30-20130210195334/@comment-3146930-20130215210744

The Underworld is Hades' domain, but Tartarus is a living thing all its' own that just drags things into itself and holds them. It is the place where the gods send their worst enemies in hopes they will never have to deal with them again. I mean Nico didn't stand a chance and I don't think Hades would do much better, not to mention that in a way Tartarus is working for Gaea and would trap Hades if he went down there for some reason. But I could be wrong as we haven't seen exactly how it works except Nico's description and Percy, Grover, and Annabeth's trip to the edge in The Lightning Thief.