People escaping slavery may have hidden inside a corpse vault.

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

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

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

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

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.

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

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

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.

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

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

A wondrous old library overlooking the nation's capital.

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

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

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

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

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

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

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

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

Home to items never before seen by the public.

The United States Capitol's former columns still stand.