The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

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

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

Home to items never before seen by the public.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

A derelict bit of transportation infrastructure hidden in the woods.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

This lonely waterfront memorial to the men of the Titanic was erected by the "Women of America."

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

The walls of the iconic dome are hollow and have a secret stairway.

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

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

The national park was once a plantation estate.

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

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

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

A celebrity from the late Cretaceous period.

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

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

Housing the National Women's Party since 1929, this historic house is now a monument to the fight for gender equality.

The most accurate timepiece in the world.

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.

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

Inside houses a vast collection of military aircraft and missiles spanning the ages.