(the erstwhile) Castle Pinckney National Monument

 from National Geographic Daily News:

Pictures: America's "Lost" National Parks

(view the entire slide show here)

Castle Pinckney National Monument
Photograph from Corbis

This Confederate island fortress in the harbor of Charleston, South Carolina, was used to house Union POWs, like these men from the Civil War's First Battle of Bull Run. During the war, Castle Pinckney was bombarded twice before falling to the Union with the rest of Charleston in 1865.

After the war, Castle Pinckney fell into general disuse and then into the hands of the National Park Service in 1933. After more than two sleepy decades, the Castle Pinckney National Monument was abolished in 1956—and still little has changed  since.

Several aborted development schemes later, Castle Pinckney, now owned by the South Carolina State Ports Authority, sits today in a state of gradual decomposition.