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

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.

This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

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

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

Behind an abandoned storefront is an example of pioneering 1930s gas station architecture.

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

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

Look closely and you’ll notice that the color changes a third of the way up the tower.

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

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

One of the largest artillery pieces in the world sits in a Washington D.C. parking lot.

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

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

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

The "lovely red Vega" of the legendary record-settling pilot.

The most accurate timepiece in the world.

Decades before the scandal, this staircase on the river was a literal "water gate."

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

This imposing Masonic temple a mile from the White House was the first public library in Washington, D.C.

The oldest continuously operating fish market in the United States.

The national park was once a plantation estate.

The United States Capitol's former columns still stand.

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