Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

An unusual stroll through gentlemen's erotica demonstrates that pornography is timeless.

The tunnel under the Capitol Building where Congress takes out its trash.

Amid the sprawling Defense Intelligence Agency Headquarters is one of Washington's least accessible museums.

Washington's top-secret Cold War-era doomsday communications tower is located in a small neighborhood park.

A historic ruin just 20 feet away from a busy highway onramp.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

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.

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

Since 1874, this macabre collection has developed into a museum devoted to Scotland Yard's mementos of dastardly deeds.

This Underground station has been a popular filming location for decades, even before it closed.

The pockmarks across the building's facade are remnants of the Blitz.

This ancient Egyptian obelisk still bears the wounds of World War I.

Its collection includes a broad array of social, geographic, graphic design, and political history.

The plunge pool is a surviving relic of a rowdy royal residence.

A decommissioned tube carriage within a London museum hosts a regular pop-up supper club.

The ritual has been performed uninterrupted for nearly 700 years.

An architectural oddity hides in plain sight on this iconic London bridge.

A Victorian school for poverty stricken waifs has been preserved as a museum to their donated education.

The last remaining example of the boatmen's perches that once dotted the south bank of the Thames.

A monument to the thrice mayor of London who according to lore made his fortune by selling his rat-chasing cat.

This unique bridge curls up like a caterpillar to let boats pass and then unfurls again to span a small canal.

Museum dedicated to the UK's first charity for abandoned children, with keepsakes of the mothers who left them behind.

A relic of Victorian grandeur hides beneath a busy South London road.