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

This former Roman Catholic Church still contains the embalmed hearts and organs of 22 popes.

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.

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

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

The final resting place of the poets Shelley and Keats.

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.

The underground sanctuary of a mysterious ancient cult.

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.

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

This intriguing Bronze Age artifact spent centuries hidden within a Welsh faerie hill.

Legend says these colorful, invasive residents are the descendants of birds released by Jimi Hendrix.

It's a mystery why these remains were found with no traces of their bodies in sight.

This ancient oak plays host to carvings of magical elves, gnomes, witches, and other creatures prominent in local folklore.

These docile dragons are ideal inhabitants for the conservatory greenhouses.

An small and easily overlooked archaeology museum chock full of Egyptian artifacts.

This massive collection of British military artifacts includes the heartbreaking personal letters of the men in the trenches of WWI.

Classic game cookery takes center stage at what's widely considered London's oldest restaurant.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.