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

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

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.

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

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.

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

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.

Home to items never before seen by the public.

This hydrochronometer was invented by a Dominican monk and is a prime feature in this garden.

This hermitage celebrates the legend of Emperor Constantine's healing from leprosy.

One of Rome's most picturesque and original bridges was also a popular choice for Grand Tour artists.

This is the oldest Roman-made bridge in the city still existing in its original state.

This statue is the only female representation of Rome's "talking statues."

These hut foundations are believed to be the residence of Romulus, founder of Rome.

A colossal aqueduct from the second century B.C., deeply hidden in a gorge on the outskirts of Rome.

A rare example of a medieval house in Rome adorned with a melancholic inscription.

Legend has it that this bronze orb, which sat atop an ancient Egyptian obelisk, once held the ashes of Julius Caesar.