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

100 years later, they're still down there.

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

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

An obscure public service from the U.S. government that redeems burnt, moldy, and soiled old greenbacks.

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

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

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.

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

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

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.

A hidden passageway now marks the site of an assassination attempt on Ronald Reagan that some say broke a 140-year-old curse.

A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

A surprising, but overlooked example of geology in the middle of the nation’s capital.

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

An off-the-beaten-path place to literally hang beneath Milwaukee's Holton Street Bridge.

A unique botanical garden housed in three giant geodesic domes.

A forgotten Cold War relic hidden in the outskirts of a Wisconsin city.

The oldest certified bowling alley in the United States is in the basement of this century-old bar.

This stunning gothic ruin was a casualty of the Indiana steel industry crash.

The NBA player's elaborate former home is now a ghost of its old glory.

What was once the second largest steel mill in the US is now a sprawling complex of modern ruins.