The span is crooked and made from two older recycled bridges.

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

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

An Industrial Revolution-era public work that purified water using nothing but sand.

The national park was once a plantation estate.

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

A derelict bit of transportation infrastructure hidden in the woods.

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

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

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

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The United States Capitol's former columns still stand.

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

The 320-year-old tree that greeted General Lafayette and rerouted an interstate highway.

A swimming hole in the historic industrial quarry that supplied the marble for the Baltimore Washington Monument.

A labor of love built by local hang-gliding enthusiasts, this ideal launch spot was outlawed by the authorities and has since been largely abandoned.

This fort down the Potomac from Washington, D.C. was once the only defensive fort protecting the capital.

The longest escalator in the Western Hemisphere takes over three minutes to ride down.

A cave on the Maryland border where civilians took refuge during the bloodiest battle of the Civil War.

An astounding collection of medical specimens, including bits of President Lincoln.

It's not hard to see why the historic Peabody Conservatory of Music's library has been described as a "cathedral of books."