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

Largest Roman Catholic church in North America.

A Spiritualist house of worship where believers communicate with the dead in the spirit world.

The first purpose-built art gallery in the United States is once again open as a center of craft arts.

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

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

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

People escaping slavery may have hidden inside a corpse vault.

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

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

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

A nostalgic bibliographic gem.

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.

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

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

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

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

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

A derelict bit of transportation infrastructure hidden in the woods.

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

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