User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-67.42.242.115-20130701044352/@comment-6068707-20130707151134

Didn't they say in the Titan's Curse I believe, that Zeus ordered one of the Gods to check up on the imprisoned titans in the underworld, and where else would they be if not Tartarus, so I don't know about that. Maybe I misread it or something, but I think it was said.