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

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

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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

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.

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

Huge masonic hall in Philadelphia with lavishly themed rooms.

This lovely pocket park is one of the most under appreciated in D.C.

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

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

Gutted during the Liverpool Blitz, this architectural husk has been left destroyed in honor of the dead.

The quiet field that inspired one of the most famous songs of all time.

What do we know about the real woman behind one of the Beatles' most famous songs?

A semi-scholarly approach to sex.

Nikola Tesla's former home contains an homage hidden from plain sight.

The world's largest Our Lady of Guadalupe statue overlooks this Ohio sanctuary.

Finally, you don't have to be a pious Swedish seaman to relax like one.

This peaceful Sikh gurudwara serves around 50,000 free meals every day within its gilded walls.

One of Catholicism's four major basilicas has a series of papal portraits that are said to predict the end of the world.

Ancient ruins hidden underneath a modern luxury hotel.

If you want to talk to the Pope you will have to stare down this surreal vision of Jesus rising from a nuclear hell.

Historic fortifications form the boundary of Vatican City, but it’s not entirely surrounded by walls.

What's left of the ruins of the 1893 Columbian Exhibition, or World's Fair, also known as the White City.