User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-4521561-20121015194512/@comment-4640327-20121016002347

Of course that Gaea prepared them a long time ago. Since the giants killed a lot of children of Athena and only a few were chosen per generation it's resonable to suppose they'd been there since WWII or so. Maybe as she rose Alcyoneus she prepared the twins to stop all demigods close to the ancient lands after the war, like a cleaning staff. They stayed there cause that's where Gaea needed them.