What was once an underground Nazi city is now Europe's largest bat reserve.

This now-defunct synthetic fuel plant was once a major cog in the Nazi war machine.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

The architect of this fanciful complex called it an "oasis for humanity and nature in a sea of rational houses."

Countless stories attempt to explain Poland's mysterious J-shaped trees.

Explore the complex ecology of this multifaceted nature preserve by riverboat.

It's always summer at the beach inside this former airship hangar.

Several of these strange cone-shaped bunkers can still be seen around the former Nazi headquarters.

Before he was forced to flee the country under Nazi rule, Albert Einstein spent his summers in this custom-built lakeside house near Berlin.

A unique, free-floating electric ferry may be the only of its kind in Europe.

The "Prussian Versailles" that is as glorious today as when it was built.

Solar observatory doubles as a daring display of Expressionist architecture, much to Einstein's chagrin.

The 1936 Summer Olympics site is a crumbling remnant of sportsmanship and a history its host country would like to forget.

The ruined remains of the airport Claus von Stauffenberg departed in his failed attempt to kill Adolf Hitler.

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

The music is over at the Ballhaus Riviera Ballroom, but the ballroom remains in haunting disrepair.

A 12th-century fortress containing some of Berlin's oldest buildings.

This conical hill was built as a testing ground for an engineer known as the Glider King.

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

Started as a kitchen garden, now the second largest botanical garden in the world.

A restaurant and observation deck built in a unique broadcasting antenna.

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

A monument to the three wars that led to German unification towers within a Berlin traffic circle.

A state-of-the-art medical facility from the Imperial era is now a derelict ruin.