Where the treasure is hidden remains a mystery to this day.
Literal
Treasure [subject-が] where [at-に] hidden-[passive-されている] [question-か] [topic-は] now still mystery is-[formal-である]
An embedded question (どこに隠されているか = 'where it is hidden') serves as the topic of the main clause. The entire question clause + か + は forms the topic, and 謎である is the comment. 今なお ('even now, still') is a literary adverb. である is the formal written copula.