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

The world's only museum devoted to the wonders of fluorescence.

Crushed coral and crystals give the sand its signature coloration.

These "anatomical machines" are only one of the many strange items in this Italian crypt.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

Pompeii's less famous neighbor is still revealing her secrets.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

Amidst the bustling streets of Dublin, there is a hushed garden that is dedicated to "All those who gave their lives in the cause of Irish Freedom."

Though they've since been renamed, these two streets hint at Dublin's darker days.

After being destroyed by IRA sympathizers, the ruins of this stately Irish manor have sat vacant for decades.

A rocky Irish outcropping with a history of wealth, conflict, and ruin.

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

The enchanting archway is the only surviving portal into Dublin's lost medieval wall.

Local legend says the remains of Father Christmas lie within the medieval grounds.

The oldest inn in Kilkenny was established by the first convicted witch in Ireland.

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

Go inside to really appreciate this 500-year-old technological masterpiece.

This community on a small Venetian island has taken to painting their houses in bright neon hues.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

The oldest operating pharmacy in the world features 800-year-old recipes.

A burial vault located under Tiber Island in the middle of the Italian capital.

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

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

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