One of our favorite things to do with friends is go explore the tunnels at Battery Steele which was built to protect Portland harbor back in WWII. Due to its location as an outer island in the bay with a clear view across the North Atlantic, Peaks Island was chosen as the principal site for Casco Bay’s defensive system. The battery was armed with two 16” guns capable of firing a 2000 pound shot 26 miles to sea!
https://en.wikipedia.org/wiki/Battery_Steele