Translation:She is travelling somewhere behind the mountains, to the sea.
Thank you, this is helpful. I was guessing it was something like this. But it is still a bit confusing, perhaps because of the English translation, which does not explicitly have a sense of "to where" associated with the mountains, as it does with the "to the sea" phrase, for which I totally get the "hez" equivalency.
Perhaps a clearer way to ask what I am trying to understand: isn't the Hungarian sentence meaning then closer to "traveling to somewhere behind the mountains, to the sea"? In the exercise, I had to translate the English sentence to Hungarian. I took the English sentence to mean that the road to the sea was behind the mountain, and thus use "mögött".
Ahh, I see your point. Yes the English version is not of a very well made sentence.
He is travelling, (with a coma here it would have been much more better, however English doesn't use comas everywhere) somewhere behind the mountains, to the sea.
So in this case it would have been clearly "mögött". But I think your version is also good. In speaking one can emphasize the focus of the English sentence by having a little break after travelling. In writing using a coma would be better.
If you use "mögött" then: Valahol a hegyek mögött utazik, (le) a tengerhez. (Hungarian(s) often use "le" if we go to a water (lake, river, sea,etc.) OR "A hegyek mögött utazik valahol, a tengerhez" OR A tengerhez utazik, valahol a hegyek mögött (but the focus here is on the location of the sea rather than the whereabout of the subject).
If you use "mögé" the solution is the correct with the "le" optionally (but that's really just an extra :D)