A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

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

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

This leafy estate is worth $18 million and is so grand it has two mailing addresses.

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

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

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

A Civil War fort in the middle of Washington, D.C. has been swallowed by a forest.

A nostalgic bibliographic gem.

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

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.

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

A derelict bit of transportation infrastructure hidden in the woods.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

A painfully obvious spy house sits right across the street from the Russian Embassy.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

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

A wondrous old library overlooking the nation's capital.

A historic neighborhood church is reborn as a psychedelic arts collective.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

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

Home to items never before seen by the public.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.