This large copper bolt represents the benchmark for measuring sea levels in most of Europe.

Model Trains and Much More in Hamburg.

A Japanese video game arcade hides in the back of a Swedish pool hall.

The remains of Alfred Nobel's first nitroglycerine factory can still be found on the shores of a Swedish bay.

The world's first combination ski slope and power plant.

The world's first motorway was used as a racetrack until the late 1990s.

This water-tower-turned-high-end-apartment building was once the site of an early Nazi concentration camp.

Home to 10,000 medical specimens including the remains of Rudolf Virchow's extensive collection of pathological specimens.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

This sculpture honors the man who was known as the "Righteous Among the Nations."

A random tree outside a Budapest hotel has become a DIY shrine to the late singer.

Budapest's Communist-era statues, banished to the suburbs.

An extensive collection of reliquaries containing the bones of saints housed in the largest Catholic church in Venice.

The small peaceful island off the Venice tourist circuit is where the city began more than 1,500 years ago.

Memorials at a Florence railway station remember the citizens who were deported from Italy and sent to extermination camps.

This strange bovine gargoyle is said to have been put in as a passive aggressive form of revenge.

This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

A large grotto built in the 16th century amidst Florence's famous Boboli gardens.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

The first green space constructed on an elevated viaduct.

Paris' fabled purveyor of exotic taxidermy and natural history curios.

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

The pioneer of psychoanalysis spent the final year of his life here, still seeing patients on that famous couch.

This beautiful Victorian ironwork masterpiece was built to keep London from smelling like poop.