This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

This museum promotes informed contraception by documenting its often secret past.

A neo-Gothic mansion now houses a wunderkammer of analog tech that hits on all the senses.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

Macabre museum devoted to historical Viennese murders.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

In the middle of a Vienna square, Roman ruins dating back some 2,000 years are on display.

The final resting place for the remains of 143 Habsburg royalty.

A magnificent collection of imperial armor and weaponry.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

A former top secret nuclear fallout shelter now operates like a hotel stuck in the Cold War.

Mummified monks and the accidentally interred, in a 17th-century crypt.

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

The hidden lair of an alchemist rediscovered in a 2002 flood.

A Jenga-style cylindrical tower of books that appears to go on forever.

A half-buried vehicle in Prague's is the latest iteration in a long-running piece by Czech artist David Černý.

Countless grotesque faces stare out from this eerie Prague art wall.

The abandoned remains of this Victorian brewery hide crumbling bowling alleys and rusting industrial gear.

A tree-like building perches on its trunk in the Steglitz neighborhood of Berlin.

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

An abandoned swimming pool decays on the outskirts of Berlin.

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

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

On the Spree bank, discover one of the last reminders of a symbolic wall that divided, and was torn down by the people.