After 400 years of being closed to the public, this monastery's underground tunnels and secret garden are now a museum.

The floor of this spectacular Baroque sanctuary is covered in memento mori.

This almost-forgotten underground dungeon housed several convicts over the years, including Caravaggio.

This subterranean necropolis holds three floors of antler-dug grave caves.

A vast Communist-era exhibition space in the Bulgarian capital.

Underneath this Byzantinian church lies a necropolis filled with ancient tombs and the ruins of previous churches.

Sofia's legislative center is continuously reinventing itself.

"Holy Wisdom" overlooks the city that became its namesake.

Eastern Bloc nostalgia, anybody?

A striking symbol of the Bulgarian capital.

A Bulgarian monument to Russia's role in World War II is constantly being painted in pop culture protest.

This historic black and white monastery is one of Bulgaria's architectural jewels.

Order a salt beef-loaded beigel at any hour of the day or night at this East End institution.

Hundreds of hidden passageways weave through the old quarters of Lyon.

Monsters and miniatures are held in equal esteem at this wondrous museum.

Lyon, France is home to two remarkably preserved Roman theaters, one dating back to 15 BCE.

For hundreds of years, automatons moving to metallic music have been marking the passage of time.

Historic medieval fortified city in southwest France.

The oldest operating roller coaster of its kind, the Montaña Suiza has been thrilling riders since 1928.

The famous polar explorer stands outside the Royal Geographical Society's headquarters.

The final resting place to 300 Victorian-era pets hides inside the city park.

London's last remaining public soapbox site has seen speeches from Karl Marx, Vladmir Lenin, and George Orwell.

A unique tribute to the millions of animals who served and died for the British military.

Never actually a tree at all, this spot was the site of London's public hangings for nearly 600 years.