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

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

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

Before founding the American Red Cross, Clara Barton had a tremendous humanitarian impact by locating thousands of missing soldiers.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

Before Nixon, "watergate" meant canals.

Home to a vast and influential collection of Shakespeareana.

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

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

There’s a special club house at the top for the bell ringers.

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

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

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

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

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

Where the U.S. government kept its actual treasure, before Fort Knox.

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

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

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

The oldest continuously operating fish market in the United States.

The national park was once a plantation estate.

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

An obscure public service from the U.S. government that redeems burnt, moldy, and soiled old greenbacks.

The house where John Wilkes Booth conspired with his co-conspirators.