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

Museum dedicated to literary greats of the Romantic Period is also the home in which Keats met his untimely end.

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

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

Installed in an act of one-upsmanship, the annual sundial in this church is also a symbolic victory for the Gregorian calendar.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

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

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

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

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.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

Architectureal fantasy quarter of Rome.

An alchemist's "magic door" stands in the middle of a Roman park.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

The recovered bones of martyrs slaughtered during The Terror are on display in this church crypt.

Jesus' footprints and the first catacombs in the world.

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

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

This modern French ossuary contains the jumbled bones of 130,000 WWI soldiers.

An ancient ossuary in Rouen.

Thousands of well-dressed mummies below a monastery.

The remains of 40,000 humans, meticulously stacked in a tiny churchyard shed.