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.

A 1,100-ton train fell through the floor in 1953. Workers got it patched up in just 72 hours.

A hidden passageway now marks the site of an assassination attempt on Ronald Reagan that some say broke a 140-year-old curse.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

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

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

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

Those with a sharp eye can find the hidden image of a German Shepherd on the memorial's Mural Wall.

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.

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

It was once the base for a series of domestic terror attacks.

Legend has it that President Grant’s frequent drinking in the lobby gave rise to the term “lobbyist.”

A celebrity from the late Cretaceous period.

The unique traffic pattern named for an influential urban planner is also known as the Pedestrian Scramble.

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

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

A much-maligned monument to teetotalism.

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

An unassuming, powerful monument north of the U.S. Capitol bears witness to the resilience of Japanese Americans during a time of grave injustice.

The remains of Martha, the last known passenger pigeon, serve as a tool to educate about conservation.

A traveling postal dog covered 48 states and more than 140,000 miles, and he lives on as taxidermy, patched up with a rabbit's foot and a pig's ear.