Hundreds of hidden passageways weave through the old quarters of Lyon.

This historic island prison holds one of the dungeons from the Count of Monte Cristo even though it is a work of fiction.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Ancient paintings known as the "Sistine Chapel of Cave Art."

One of the greatest maze designer’s greatest mazes.

This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

A treasure trove of Renaissance-era funerary art in a Roman church built on top of Nero's grave.

Brass cobblestones mark the places where individual Holocaust victims were taken from their homes.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

One of seven Jewish catacombs in Rome and one of only two open to the public.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

One of the most important yet well-hidden museums dedicated to the history of medicine.

In Rome’s Jewish ghetto, a 200-year-old bakery still makes the neighborhood’s historic sweets.

A forgotten "city of water" underneath the famous Trevi district.

A collection of objects supposedly singed by the hands of souls in purgatory.

This ritzy Italian market is built around a statue of an unforgiven heretic who was burned for his belief in an endless universe.

A fairy tale castle in modern Rome.

Jesus' footprints and the first catacombs in the world.

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

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

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

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