People escaping slavery may have hidden inside a corpse vault.

A brass and marble scale model of the solar system embedded in the concrete.

‘Bush the old fire dog died of poison July 5th, 1869. RIP.’

A small token of gratitude from freezing cold journalists who were given a warm haven while covering JFK's inauguration.

This stately mansion, built in 1787 by America's first Secretary of the Navy, is rumored to be one of the most haunted buildings in Washington, DC.

The site of the climactic scene from the classic horror film is now a historic landmark.

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

A derelict bit of transportation infrastructure hidden in the woods.

This forgotten boat elevator was an engineering marvel in its heyday.

This Moab institution functions as both a rock store and a small natural history museum.

Mysterious installations bring levity and light to a sometimes stodgy city.

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

This brick-lined pedestrian walkway was once a railway tunnel used during the Civil War.

A line of blue paint marks the route of the inaugural parade.

Part of a WWII submarine is lurking outside the Washington Navy Yard parking lot.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

A much-maligned monument to teetotalism.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.