Venice's "Island of the Mad."

This abandoned train tunnel has been the subject of countless ghost stories for years.

A particularly creepy abandoned hospital that sparks interest for lovers of the supernatural.

St. Mary's Asylum was isolated and abandoned before being converted into a Bed and Breakfast.

The decaying ruins of Wales' first asylum for the mentally ill.

This abandoned asylum in Wales might look haunted, but it's still beautiful.

An abandoned asylum incorporated into a college campus.

Clusters of abandoned buildings hint at the facility's 144-year history with psychiatric health.

An abandoned asylum where patients have been forgotten but their possessions remain.

This abandoned asylum was once a state of the art facility before devolving into one of the most deadly mental institutions in American history.

A subterranean world of tunnels links several buildings of this rare 19th-century asylum.

There's a labyrinth of manmade tunnels stretching for over an acre beneath Bristol.

A medieval cave used to shelter local hermits now holds nearly 200 old Quaker gravestones.

A homemade anatomical collection of medical anomalies and accidental catastrophes.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

This iconic fortress was once the crown jewel of Haiti's national defense.

The Avenue of Mausolea is a series of grand palatial tombs.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

Burial grounds for more than 1,000 bodies deep under the modern town of Rabat.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

This castle played host to the largest great hall found in medieval Central Europe.