Fort Mifflin, originally called Fort Island Battery and also known as Mud Island Fort was commissioned in 1771 and sits on Mud Island (aka Deep Water Island) on the Delaware River below Philadelphia International Airport. During the American Revolutionary War, the British Royal Navy bombarded and captured the fort as part of their conquest of Philadelphia in autumn 1777. In 1795 the fort was renamed for Thomas Mifflin, a Continental Army officer and the first post-independence Governor of Pennsylvania.

The United States Army began to rebuild the fort in 1794 and continued to garrison and build on the site through the 19th century. It housed prisoners during the American Civil War. The army decommissioned Fort Mifflin for active duty infantry and artillery in 1962. Curators, caretakers, renovation crews and even tour guests have made eyewitness reports of hearing screams and gunfire, seeing the images of soldiers and prisoners at the Fort. The Ghost Finders are ready to investigate.