User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7594114-20130811072725/@comment-3146930-20130812001121

I think it is, but never really all at once. At the beginning of The Son of Neptune, Mars explains that souls are escaping and monsters are not returning to Tartarus because Thanatos was escaping. Later when Thanatos was freed, he said he could not close the Doors of Death because Gaea was too powerful, but he would at least prevent more souls from escaping and monsters would die again.