A playground the size of a city block is a modernist monument to civic values and public housing.

The palace window through which Protestant nobles flung two Catholic regents, thus sparking the Thirty Years' War.

Despite attempts by local police to discourage it, Beatles-inspired graffiti has been a means for artists to air grievances for decades.

A giant interactive periodic table with real samples of the elements.

The only natural cave bath in all of Europe.

A peculiar clock commemorates a historic victory won by ringing the noonday bell an hour early.

Europe's second largest ossuary wasn't discovered until 2001.

An extraordinary museum devoted to the pop art legend created by Slovak artists.

This region of Slovakia boasts some of the most plentiful opal mines in the world.

One of the few cold geysers in Europe, its eruptions are powered by carbon dioxide.

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

After local authorities changed the design, the building's architect, Le Corbusier, wanted nothing to do with it.

In the summer of 1957, a massive architectural exhibition showed off the future of urban living in this German neighborhood.

This monumental work of art symbolizes the coming together of a previously divided city.

Metro station with a claim to rock n' roll fame.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

An abandoned NSA listening post on Berlin's "Devil's Mountain."

Berlin's old airport.

A towering monument to war, located on a beautifully peaceful hill.

The skeletal frame of a decommissioned gas-pumping station offers breathtaking views of greater Berlin.

A wunderkammer of mysterious objects and stories.

Cold war icon and onetime "hole" in the iron curtain.

This unique structure allowed boats to bypass underwater mine barriers in the channel to enter the harbor.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.