One of the most influential presidents in U.S. history wanted only this plain, elegant monument as his lasting memorial.

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

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

Go take a picture with the inventor of the daguerreotype photographic process.

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

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

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

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

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

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

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

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.

Rumor has it the uncarved lump behind the three famous suffragists is reserved for the first woman president.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

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

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

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

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

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

The most decorated dog of World War I is preserved in the Smithsonian.

Giant acorn-shaped 19th century gazebo from which suburban Washingtonians gazed upon the original "silver" spring.

A girls' boarding school inspired by the Chicago World's Fair, once abandoned, now restored to strange and scenic glory.

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