This simple stone memorial remembers a secret, Geneva Convention-defying interrogation base from WWII.

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

America's first federal monument.

World War II-era bunkers are hidden under the beaches of this picturesque park, which is still scattered with wartime watchtowers.

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

There was a top secret plan during World War II to seal a group of soldiers inside the rock.

Several times a year, during low tide, two pairs of historic train tracks are revealed on this New Jersey beach.