Originally Posted by Rebell L
Only problem is, I can't think of a good reason why the magical world would hide and how they'd keep everything contained (before they fail and everything goes public), or how the population would react exactly.
Have you considered a mixture of simple xenophobia and the faster population growth in humans?
Back in the day, when magic was more open (resulting in myths, fables and fairy tales) the "magic people" had more than enough room to avoid the "icky" mortals. Can be any reason why they hate us. Maybe our "ways" are repulsive, maybe we are the equivalent of mayflies and just as annoying, maybe we simply "stink".
Fast forward to the cure/prevention of diseases and better agricultural techniques and BAMM! more and more people in more and more places. If you were an immortal, you packed your stuff and looked for more secluded places.
If you need an explanation why they ALL went into hiding, just have some kind of United Magical Nations that made the decision that in order to protect all races, everyone had to hide, even if some didn't mind humans as such.