A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

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

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

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

This hill made entirely of ancient Roman jars was simply a garbage dump to the people of the time.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

Once a path for Jewish funeral processions, this unassuming alleyway is rich in local lore.

A memorial in the Tiergarten is a reminder of the suffering inflicted on homosexuals by the Nazi regime.

Berlin's oldest surviving academic architecture hosted decades of animal dissections.

Berliners converted a bombed-out, disused lot into a community-run urban farm with its own cafe and bar.

Residents of Berlin take their coffee and cake in one of the city’s most restful spaces: a cemetery.

Super-heavy Nazi architectural experiment.

A wunderkammer of mysterious objects and stories.

An abandoned saucer-shaped monument in the Bulgarian mountains.

The entrance to this Frankfurt subway station looks like an old tram car crashed into the concrete sidewalk.

Hundreds of notes give thanks to the patron saint of lost causes.

A concrete Picasso in the middle of a student housing complex on the outskirts of Stockholm.

A king's bloodstained shirt is among the gruesome collection of artifacts displayed below Stockholm's Royal Palace.

This grisly exhibit houses skeletons and weapons from a devastating 14th-century clash.

It's always game time at Stockholm's only museum devoted solely to video games.

This gem from the industrial age helps visitors and residents alike traverse hilly Stockholm.

A golden plaque marks the site of the mysterious unsolved assassination of the former Swedish prime minister.

Stockholm's deepest subway station is also a stunning ecological wonder.