This World War II bunker now survives as a documentation of the Nazi regime and war crimes.

Super-heavy Nazi architectural experiment.

This Berlin island is home to a flock of free-roaming peacocks, as well as a historic ivory folly.

A narrow passage next to a cafe leads to a courtyard exploding with murals, tags, and paste-ups.

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

A whimsical and twisted animatronic monster tour beneath the streets of Berlin.

Originally a leper colony, the island is now an Armenian monastery that's home to an incredible manuscript library and eclectic museum.

The now-abandoned tunnels were built for the transcontinental railroad on the route where the first wagon train entered California.

A Roman spa buried deep beneath the City of London.

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

This secret garden has bloomed into London's most unique hub of urban creativity.

A relic from one of London's worst 19th-century slums.

The secret wartime bunker built on the platforms of an abandoned Tube station.

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

An unsanctioned shrine that sprung up outside the late singer's home is causing a stir in London's Highgate neighborhood.

This studio was home to David Bowie's first single, even if it did turn away the Beatles.

A portly statue of a golden boy commemorates an unusual cause of the Great Fire of London: the sin of gluttony.

Black cat statues guard this temple-like Egyptian revival factory.

A network of concealed passageways lies within a busy London Underground station.

This elaborate Byzantine-style building was designed for a not-so-pretty purpose.

Residential building on the Regents Canal is a breeding ground for creativity and innovation.

This hidden green space is the city's second largest conservatory.

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

The monster at the center of the Darwin Center Spirit Collection.