Jesus' footprints and the first catacombs in the world.

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

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

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

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

2300-year-old Roman road connecting the empire.

The final resting place of the poets Shelley and Keats.

The underground sanctuary of a mysterious ancient cult.

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

An eccentric artist's abode in this small Italian town has been transformed into a surreal temple of mosaic madness.

A black cat protests the City Guild in Riga's Old Town.

3,000 market stands in five former Zeppelin hangars.

An old hill fort covered in over 100,000 crosses is a symbol of Lithuanian nationalism and beliefs.

A cathedral resurrected from the ashes of one of Europe's greatest urban fires.

An abstract monument to a symphonic composer.

A collection of collections ranging from dentistry to minerals.

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

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

The remnants of an early American attempt at missile defense.

Achill's most recently abandoned village dates back to to the Age of Enlightenment.

A nun spent 16 years painting the stunning Celtic mural that covers the interior of this WWI memorial.

More than 5,000 artifacts donated by the people of the city, plus a room entirely devoted to the band U2.

This ancient Irish tower could easily have stood in for Rapunzel's prison.

The mummified heart of a saint lives on in this church—after a brief hiatus when it was stolen.