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

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

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

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

The "Key of Keys" for Washington, D.C. and the apocryphal landing place for the military campaign that started George Washington's career is now at the bottom of a well.

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.

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

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 National Park Service’s best-kept secret with views that rival any in D.C.

A much-maligned monument to teetotalism.

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

This statue of one of baseball’s greatest pitchers looks like something out of a sci-fi horror movie.

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

This statue of the "father of television" stands prominently in the United States Capitol.

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 promising candidate for the lost rock laid by George Washington was unearthed, but the hunt is still on.

Home to items never before seen by the public.

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.

An easily overlooked memorial to a Ukrainian famine-genocide that killed over 4 million people.

Railroad officials in the early 1900s sought to spare travelers the sight of Roman soldiers’ private parts.

A 1,100-ton train fell through the floor in 1953. Workers got it patched up in just 72 hours.

An unassuming, powerful monument north of the U.S. Capitol bears witness to the resilience of Japanese Americans during a time of grave injustice.