This abandoned dream castle and its cemetery look out over the city that once inspired their creation.

A secret greenspace hides atop a Capitol parking garage.

The first memorial dedicated solely to the Black troops who fought for the Union.

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.

The grand marble structure next to the White House is Andrew Carnegie's temple to Pan-American diplomacy.

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

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

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.

"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!”

A bas relief commemorates the "Nuns of the Battlefield" who cared for soldiers on both sides of the conflict.

When it comes to handsignals (and colonialism) rock always beats scissors.

100 years later, they're still down there.

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

A small triangle of DC grass is the final resting place of one of Sonny and Cher's songs.

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

Four bald cypress trees in Lafayette Square, across from the White House, mark the southern edge of a 100,000 year old cypress swamp.

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

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

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

This grand gothic brewery has been pumping out suds for over a hundred years.

The United States Capitol's former columns still stand.

The Pentagon is so large that it was planned like a city, complete with internal highway infrastructure.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.