It's a fairly common trope in Urban fantasy (albiet hardly universal), that the magic is a secret. There's a hidden, secret world where all the monsters and wizards and stuff live. Sometimes there isn't even that, and it's all very much the same world, just conspicuously out of sight.
This style of worldbuilding is a lot of fun, and the juxtaposition between a fantastical and mundane world is a big draw for a lot of works.
The issue, though, is justifying this from a worldbuilding perspective. I've become interested in writing one of these settings myself, so I was hoping to ask...
What justifications have you seen for a secret world, and which ones were your favorites. What worldbuilding concessions did they explore? My current idea is that the magic system itself rejects the idea of being known and understood. That if too many people know definitive things about what magic is and isn't, that the power of magic will fade worldwide. Hence, all magical beings work together to keep it secret. Even the magic humans. Have you seen this approach before? Does it sound plausible? Did it work?