The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

More sculpture than building, this architecturally significant temple was built on the spot of Saint Peter's crucifixion.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

A Nesting Doll of Churches.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

Cave shrine on Monte Pellegrino.

Thousands of well-dressed mummies below a monastery.

Every inch of this chapel is covered in craftsmanship from across the world.

A collection of grotesques lines the walls of this Italian villa.

Italian cave with exceptional acoustics and an unsavory history.

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

Microscopic scenes; some call it novelty, some call it art.

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

Macabre astrological automaton clock dating to the late 1400s.

Looming communist pillar with a Czech twist.

Pilgrimage site for star-crossed lovers.

A marvel of Byzantine engineering under the streets of Istanbul.

Roman ruins top white travertine terraces formed by ancient hot springs.