Medieval irrigation channels built into the sides of mountains provided Valais with water for hundreds of years.

For centuries no one knew there were hundreds of bones and precious artwork hidden beneath this Swiss church.

This ossuary is more than five centuries old and is heralded as one of the finest in Switzerland.

Indents in the pavement are all that remain from a grisly death machine in Paris.

France's most famous cemetery, with some of its most curious tombs.

Stripes of human ashes cover a back lawn in the most sought-after burial grounds in Paris.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

A subterranean shelter remains intact under a busy Paris railway station.

A unrivaled and rather horrifying collection of wax dermatological models.

This 170-year-old Swiss guesthouse built into the side of a mountain is best accessed by a cable car.

At first glance, this area seems like just another city park, but the lush landscape hides a dark past.

A church of bones, decorated with 40,000 human skeletons.

Poland's oldest salt mine has nearly everything one needs to live underground forever.

An underground city of salt.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

Created as a "model concentration camp" to hide Nazi atrocities from the international community, this former camp-ghetto now holds a museum and memorial honoring the lives lost within.

The mystery of the Putim Ossuary involves a tricorn hat.

This network of conjoined cellars was riddled with traps and supposedly allowed the townspeople to pretend their homes were deserted.

These 17th-century mummies are no longer naturally preserved, but remain on display today.

Mysterious painted skulls in Moravia.

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

Thirty creepy ghosts now inhabit this decaying 14th-century church.

Europe's second largest ossuary wasn't discovered until 2001.

Underneath this small church are 15,000 bones, arranged by an obsessive academic.