A subterranean house of worship carved right out of the bedrock.

This irregularly shaped sea fortress has defended not one but three different sovereign states.

An alien church provides much needed silence in the midst of a Finland shopping center.

Architecturally stunning library serves as the hub of Finnish scholarly life and culture.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The oldest stone house in Paris was built by its most famous alchemist.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

An enormous and lavish Gothic church in Czechia.

A spiral staircase twists to the top of this unusual Czech lookout tower.

The world's greatest slide descends 18 stories from this looping mountaintop viewing platform.

This doorless 20th-century elevator doesn't stop or slow down.

Thirty creepy ghosts now inhabit this decaying 14th-century church.

A steampunk-styled three-floor factory churning out art, activism, and electronic beats.

The first museum in the world devoted to sexual gadgetry.

Walk through Czech history in Prague's largest graveyard.

Around 100,000 bodies are buried below the medieval headstones, many of which mark graves with multiple bodies stacked up to 12 deep.

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

A church of bones, decorated with 40,000 human skeletons.

The stones that traveled from Spain to eventually become a holy place for San Francisco's nature worshippers.

This picturesque island graveyard is the final resting place of dozens of notorious pirates.

The centuries-old trees lining both sides of this dirt road may become Madagascar's first natural monument.

A strange house built only at night, with rumored mystic inspiration.