Fort Knox, Maine
Fort Knox is about an hour east of Bar Harbor and Acadia National Park. Fort Knox was built from 1844-1869. The fort was decommissioned and sold to the state of Maine in 1923. The fort has very dark passage-ways running through it's lower levels. There are 8 straight corridors that encircle the inner and outer fort structures. Visitors are free to walk around in the passage ways. There is no artificial light so it's very dark in these tunnels. Not many people were visiting the fort when I was there and only one other person ventured down into the lower corridors, so I was able to set up a tripod and take some (in my opinion) very interesting pictures. Access to the fort and parking are free. There's a nice view of the Penobscot Narrows and bridge from the upper levels of the fort.