Pyrenees
These are the mountains between France and Spain. It’s neatly self-contained – running across the border, not quite reaching the Channel in the north or the Mediterranean in the south.
The country of Andorra is on the border, right in the middle of the Pyrennes. (Which, I maintain, was the model for Genovia in The Princess Diaries.)
Why I Looked It Up
I didn’t note it, but I think it was in The Confessor, since the characters spent quite bit of time in those two countries.
Also, for the record, I knew this was a mountain range in Europe, I just couldn’t have pointed it out on a map.
Postscript
Added on
In February 2024 I was in Barcelona for a conference. I stayed an extra day, rented a car, and drove into the Pyrenees to visit Andorra. I spent a day up there. It was pretty.