This massive library holds what was once the largest book collection in the world.

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

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

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

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

Memorial to the lost dead of Gdansk, whose resting places were destroyed during World War II.

This 700-year-old structure is one of the most unique architectural and historic monuments in Gdańsk.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.