A small etching on the white marble parapet was written in runic script by a Viking mercenary.

An Art Nouveau gem hides beneath the layers of city grime obscuring this once-magnificent building.

A section of the long chain that blocked enemy ships from entering Constantinople.

A surprising and unconventional mosque hidden in the subsoil of Istanbul.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

The hidden lair of an alchemist rediscovered in a 2002 flood.

The remains of this deep, dark mine have left behind a haunting rocky warren of tunnels.

This 30-foot pile of rocks may be the grave of a mythological Irish queen.

One of Ireland's most infamous prisons for rebels including men, women and children.

A museum dedicated to the Spanish occult in a town that was terrorized by the Inquisition.

This Navarrese cave's history of witchcraft and pagan ritual is not just embraced, but recreated each year.

Spain's unique Mosque-Cathedral has been a place of worship for both Muslims and Christians throughout its history.

A micro-communist utopia in Spain with no police, no mortgages and full employment.

A curious carving hides on a marble column in the Mosque-Cathedral of Córdoba.

The mysterious remains of a martyred teenage saint are venerated inside a small country chapel.

This enormous censer uses a pulley system installed over 400 years ago.

An accidental trick of light casts the perfect shadow of a pilgrim of the Camino de Santiago against the cathedral wall.

Thanks to an acoustic anomaly, this unassuming stone bench has heard one hundred years of secrets.

The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

Step inside the first house designed by Catalan architect Antoni Gaudí, considered to be one of the first Art Nouveau buildings.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

Elevated freight railway turned wildly successful urban park.

Board a mine car and descend the #190 slope 300 feet underground into the Clark Vein of coal.

The sole remaining outpost of a once-thriving South Carolina chain still sells “pic-a-nic” baskets.