A former mental hospital founded in the 1930s by a eugenicist who advocated for the forced sterilization of patients.

The Cold War-era "underground Pentagon" is a sprawling subterranean hideaway for the Defense Department.

Three abandoned missile silos are hiding in plain sight behind a suburban development in Maryland.

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

Venice's "Island of the Mad."

Mummified monks and the accidentally interred, in a 17th-century crypt.

Thousands of well-dressed mummies below a monastery.

The oldest of Venice's plague quarantine stations, designed to keep the medieval city safe from disease.

Originally a leper colony, the island is now an Armenian monastery that's home to an incredible manuscript library and eclectic museum.

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

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

The Vatican's secret porn-festooned bathroom was painted in erotic murals by one of the greatest artists of the Renaissance.

If you want to talk to the Pope you will have to stare down this surreal vision of Jesus rising from a nuclear hell.

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

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

Jesus' footprints and the first catacombs in the world.

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.

The underground sanctuary of a mysterious ancient cult.

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

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

Vast network of air raid tunnels beneath Lombardy's capital.

One of a series of no-purpose kinetic artworks by Swiss sculptor Jean Tinguely.

Site of a famous 19th-century transcendentalist utopian community, now abandoned.