Yes, there is a Fort Knox in Maine and it's not full of gold.
The Fort Knox Historic Site, in Bucksport ME, features one of the best-preserved examples of coastal defense fortifications constructed in the mid-1800s. It is America's first fort named after Major General Henry Knox, the same General that the Fort Knox in Kentucky is named after. While this fort never saw any action (did not fired a single shot) is was still interesting to see this Fort and hear about it's history.
The Fort is situated on the Penobscot river at the "Narrows" with the impressive Penobscot Narrows Bridge right next door. The Penobscot Narrows Observatory is the tallest bridge observatory open to the public in the world. A quick ride to the top gives an impressive view of the Penobscot River, Fort Knox, and Bucksport.