One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.

An honorary saint of the people, Maria Adelaide became an incorrupt corpse, the clothes intact, exhaling “a strong scent of roses.”

Houses are tucked between, on, and underneath giant boulders.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

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

Founded as a refugee camp, now a popular tourist spot.

Crumbling necropolis set high above Fez.

Traditional 11th-century outdoor tannery.

One of the most spectacular canyons in the world.

Man-made cave with enigmatic carvings, some say made by the Knights Templar themselves.

The biggest and deepest Cold War bunker open to the public can be found in southeast England.

A marvel of Byzantine engineering under the streets of Istanbul.

19th century tunnels burrowed under Liverpool.

A Nesting Doll of Churches.

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

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

The curious underground history of keeping Paris clean.