A salty cave excavated by elephants and the location of a deadly disease vector.

An abandoned facility that has become a place of ritualistic satanic worship, or so rumor has it.

Long abandoned underground complex to keep royals safe from a nuclear war that never came.

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

This stately estate house features a tower covered in a bright, modern mural.

Ruined 11th century abbey built by King David I in Edinburgh, Scotland.

One of Scotland's most important modern structures, it just made the '100 Most Endangered Sites' list.

Limestone grottoes turns everyday objects into stone.

One of the world's largest salt caves is filled with fragile salt sculptures, salt rivers, and huge salt megadomes.

Shakespeare, David Bowie, Dr. Who, the Saxons, Druids and Romans are all claimed to have all made their mark on these massive man made tunnels.

Man-made cave with enigmatic carvings, some say made by the Knights Templar themselves.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

Charnel house at the foot of Mount Sinai.

Mysterious painted skulls in Moravia.

The mortal remains of an estimated 70,000 people lie in the basement of a monastery in Lima, Peru.

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

5,000 skulls in memorial to those killed by the Khmer Rouge.

The personal charnel house of Anton Bruckner.

Artfully arranged bones of 5,800 Austrians.

A church filled with the bones of hundreds of virgins martyrs—or so they say.

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

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

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