Perhaps deep underground or underwater. We've explored most of the landmass on Earth and some of Antarctica, but we know less about the Deep Ocean below the oceans' surface than we do Space even. And the Deep Underground inside extensive cave networks and subterranean structures, we know even less so. For instance, there are thousands of identified but unexplored caves in the state of Missouri alone, just think of how many there are in the whole world in China, Russia, the Middle East, Europe, Africa, Australia and so on.
Both have natural barriers that seems to limit pretty much all of us from exploring it except the very most determined with the very best skills and equipment. In the Deep Ocean, it's as dark or even darker than space, intense barometric pressures (every 33 feet, the pressure increases by 1 whole atmosphere at sea level), no oxygen of course like space, and the land mass to explore is far vaster than the land above sea level.
In the Deep Underground, you have to brave not only barometric pressure, but intense heat from lava, extreme darkness, lack of oxygen/poisonous gas clouds, cave collapses, tight spaces, and so on to fully explore it.
If there are hidden cities of magic out there, hidden to keep magic itself hidden from us out here on the surface because perhaps we can't handle it, I would see them likely being hidden in the Deep Underground like the mythical Kingdom of Agartha or hidden in the Deep Ocean like a city of Atlantis.