The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

Ireland's most remarkable collection of these curious cone-shaped houses is worth the buzz.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

When it comes to handsignals (and colonialism) rock always beats scissors.

Washington's top-secret Cold War-era doomsday communications tower is located in a small neighborhood park.

The oldest building in the District of Columbia was preserved because of a mistaken connection to George Washington.

An overlooked memorial honoring the local Washington residents who died in World War I.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

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

There’s a hidden military meme engraved on the World War II Memorial.

A strange egg in a mucky bog of western Ireland marks the spot of an aviation milestone.

You can walk through the first Boeing Air Force One, which famously carried JFK's casket from Dallas back to D.C.

This small museum gives visitors a chance to soak up the history of the "sponge capital of the world."

Perhaps the most scandalous statue of America's first president.

The rogue roots are gradually consuming some of the historic marble grave markers.

After a water tower painting was accused of being an ad, a simple fix was made that gave the city a motto.

The world's longest known cave system.

This replica of the Greek structure is in the heart of Nashville.

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

The historic home still shows the scars from a little run-in with the British in 1813.

The school tested mustard gas for the U.S. Army during World War I.

Washington's iconic carousel has a nice piece of Civil Rights history.

One critic described it as "middle Mussolini, early Ramses, and late Neiman-Marcus." Another called it an architectural "natural disaster."