This former maximum-security prison is creepy, sobering, and supposedly haunted.

The highest point in Tennessee, Clingmans Dome bears witness to the ravages of one type of insect.

In paradise, feral pigs have claimed an island all to themselves.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

A forgotten "city of water" underneath the famous Trevi district.

This limestone outcropping was once a ceremonial site for ancient Native Americans.

A natural sandstone bridge hidden in the Sewanee forest.

One person's lost luggage is another person's treasure.

An ancient cave system in Tennessee’s Appalachian foothills converted to a 1,200-person music venue.

This mansion turned restaurant was once a pitstop for Al Capone during his travels.

The sister plane of the C-123 shot down during the Iran-Contra Affair has found a new purpose in retirement.

In the fictional world of Kcymaerxthaere, celestial wonders marked a portal to a powerful route of travel at this site in Madrid, Spain.

A whimsical brother of the nearby Fallen Angel statue takes the falling part quite literally.

This massive fiberglass Gulliver turns every visitor into a Lilliputian.

A once-secret library of Freemason history in Catalonia.

The earliest known map to show America, discovered by accident at a Paris bookshop.

The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

A completely underwater art museum sends a stark message about humans' impact on the environment.

One of the most photographed buildings in a city known for its Modernista architecture is an abandoned squat.

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

The final resting place of the great navigator, or maybe his brother.

Over two centuries old and counting, Madrid's massive botanical garden contains one of Europe's only desert environments.

This former Baroque palace on the historic Calle Montcada is a rare architectural gem and was once the meeting place of an aristocratic scholarly society.