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

A nostalgic bibliographic gem.

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

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

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.

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.

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

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

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

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

America's Most Wanted's set resides in this tribute to the history of crime and punishment.

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

Bar full of oddities, specimens, artifacts and homages to the great dime museums of the past.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

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

A tiny piece of the Moon is embedded in this stained glass masterpiece.

The site of the climactic scene from the classic horror film is now a historic landmark.

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

The United States Capitol's former columns still stand.

Giftshop catering to cryptozoologists and anyone with a sense of humor.

The studio built by fascists in the hopes of "reviving the film industry" with the occasional propaganda film opportunity thrown in.