This unattributed sculpture in Amsterdam is hiding in plain sight.

The largest flower auction house in the world.

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

This ritzy Italian market is built around a statue of an unforgiven heretic who was burned for his belief in an endless universe.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

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

Rome's semi-secret peephole vista is also a former Crusader stronghold.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

Museum dedicated to the terror regimes of Hungary.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

A 19th century artistocrat's mansion, turned into a library, hidden in a modern library.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

Lithuania's outdoor sculpture trail of wooden folk art.

An old hill fort covered in over 100,000 crosses is a symbol of Lithuanian nationalism and beliefs.

A black cat protests the City Guild in Riga's Old Town.

3,000 market stands in five former Zeppelin hangars.

This abandoned secret city is a crumbling wonderland of Soviet ruins.