This massive library holds what was once the largest book collection in the world.

Notorious artist squat renovated into legal studios.

Abandoned railway line circling the city of Paris.

France's most famous cemetery, with some of its most curious tombs.

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

The martyr's wax-enhanced skeletal remains lie preserved in a glass case in the Santa Maria della Vittoria in Rome.

One of several skulls claiming to belong to the beheaded St. John the Baptist is on display at this Roman church.

This beautiful park on the outskirts of Rome protects the ruins of two colossal ancient aqueducts.

Jesus' footprints and the first catacombs in the world.

Architectural monsters are devouring this palace's every door and window.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

2300-year-old Roman road connecting the empire.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

The underground sanctuary of a mysterious ancient cult.

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

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

An alchemist's "magic door" stands in the middle of a Roman park.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

Architectureal fantasy quarter of Rome.

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

This wrecked airplane has been sitting on a black sand beach since the 1970s.

The Icelandic Sea Monster Museum explores the area's history of aquatic cryptid sightings.

An encyclopedic collection of mammal penises.

Walk in the footsteps of the gods and straddle two tectonic plates.