The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

Just outside of Nairobi lies a large estate surrounded by endangered Rothschild giraffes, who have been fed from the window by Mick Jagger and Walter Cronkite.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

The only rotating bar in New Orleans has been slowly spinning since 1949.

Dine on food tied to Viking history and legend at this subterranean Stockholm spot.

This Prague museum collects brutalist relics and memories from the country's former Soviet rule.

Two stunning 17th-century libraries and a cabinet of curiosities.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

A centuries old castle atop a dormant volcano houses museums devoted to both women and witches.

An Austrian house of bones filled with hundreds of intricately painted skulls.

Artfully arranged bones of 5,800 Austrians.

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

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

The beautiful baroque library of the Hapsburg empire.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

A fascinating insight into daily life in the Eastern Bloc.

A wunderkammer of mysterious objects and stories.

A museum dedicated to the history of a terrifying secret police is now held in their former headquarters.

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

An eerie, derelict amusement park in Berlin.

A haunting collection of the vanished and disappearing natural world.