User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3146930-20121014230006/@comment-86.128.165.111-20121016192648

The only somewhat logical reason I can suggest is that Gaia is overall more powerful than Thantos, of course she's still in a slumber, but she's possibly powerful enough to overpower him and keep the underworl doors open. Or maybe she's influencing and underworld god, like Hecate to keep them open.