The verdant mossy ruins of a Franciscan monastery lie in the remote Sintra hills awaiting exploration.

This Industrial Age architectural and engineering gem helps visitors and residents alike traverse Lisbon, “The City of Seven Hills.”

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

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

Underneath the city that invented pilsners are a series of historic tunnels that have been storing beer for centuries.

The mystery of the Putim Ossuary involves a tricorn hat.

Folklore says this medieval fortress was plopped atop a portal to hell to trap the demons below.

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

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

Mysterious painted skulls in Moravia.

The deepest sinkhole in Central Europe comes with a grisly tale and an unusual nickname: “The Stepmother Abyss.”

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

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

An old asylum graveyard that's been abandoned for half a century attracts those seeking a dark thrill.

Buried five stories underground, this Soviet bunker is packed with gas masks and Cold War paraphernalia.

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

A collection of anatomical samples from one of the first anthropologists to suggest that humans reached North America by crossing the Bering Strait.

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

The anatomical-pathological collection of a man who changed surgery.