These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

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.

A celebrity from the late Cretaceous period.

You could be standing at the site of one of D.C.'s most fatal tragedies and not even know it.

An Industrial Revolution-era public work that purified water using nothing but sand.

A super-sized promotional trick that is now a D.C. landmark.

A computer bug so primitive it was an actual insect.

There's a crumbling old mansion inside the Smithsonian National Zoo.

The famous abolitionist’s preserved estate is one of Washington's finest monuments to its great Black citizens.

The national park was once a plantation estate.

When it opened during the final years of the embargo, all the drinks and cigars were free.

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

A haunting tribute to pioneering photographer Clover Adams.

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

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

People escaping slavery may have hidden inside a corpse vault.

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

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

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

A derelict bit of transportation infrastructure hidden in the woods.

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

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.