User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-17724486-20130926225836

Final mystery location released on Rick's twitter/tumblr. Here's the link: http://rrriordan.tumblr.com/post/62363350339/final-mystery-location-the-argo-ii-will-dock-here

SPOILER:

It's Valleta, Malta...compared to the other locations he's given out, one question remains:  how the heck do they end up in Malta??