Prospero's Children — Jan Siegel

It began ages past in fabled Atlantis, when a mad, power-hungry queen forged a key to a door never meant to be opened by mortal man--its inception would hasten her own death and the extinction of her vainglorious race. For millennia the key lay forgotten beneath the waves, lost amid the ruins of what had been the most beautiful city on Earth. But however jealously the sea hoards its secrets, sooner or later it yields them up. Now, in present-day Yorkshire, that time has come. And for young Fernanda Capel, life will never be the same again...