At the crest of' is a phrase used to describe reaching the highest point or peak of something. It can be used in reference to a hill or mountain where one would be at the top or crest of it. It can also be used in reference to a wave or other natural phenomenon where one would be at the highest point of it. The term ATOP is a shorter way of saying 'at the crest of' and is often used in place of the longer phrase.