Why am I developing a circular indentation on my forehead? (Photo)

In the last two years I have developed a circular indentation on my forehead, It is hollow as if a cup has been pushed into my forehead. The indentation is becoming deeper and more noticeable as time goes by. What is the cause and is there a procedure that can fix it? I have not done anything to my face chemically or surgically, it is very strange that I would develop a circular wrinkle (if related to getting older).

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