Thunder Bay Island sits just three miles East-northeast of the north point of Thunder Bay, and thirteen miles from the harbor at Alpena. This 215-acre limestone island is the outermost of a group of islands connected to the shore by a shallow rocky shoal. As such, it represented both a significant marker for Northbound vessels making the turn toward the Straits, and stood ready to chew the wooden hulls of vessels unlucky enough to stray too close to its rocky shores.