Historic medieval fortified city in southwest France.

The grand complex of buildings that comprise Liverpool's Exchange Flags includes a vast World War II bunker that has been frozen in time.

This unusual hostel provides a glimpse of life in a city under siege.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

An engineering error ensured this cavernous 350-foot-long subterranean thoroughfare was never used as intended.

This chimney is the oldest free standing industrial chimney in Britain.

The sanctuary seems to either float atop the water or sink into the dirt, depending on the water level.

One of the largest ossuaries in Spain is packed from floor to ceiling with bones.

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

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

The site of the first battle of World War II.

Visit an imposing 13th-century Transylvanian citadel lurking not far from the region's more heavily touristed attractions.

This Vietnamese hell cave lets visitors take a walk through a devilish underworld full of demons.

A relic of the Cold War tucked away in the forested hills above Bratislava.

Thousands of well-dressed mummies below a monastery.

This old Victorian bathhouse is fed by an ancient spring.

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

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

Natural history museum celebrating biodiversity with a parade of taxidermy animals.

The legendary cliffside castle of three-time Voivode of Wallachia, Vlad the Impaler.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

Underneath this Byzantinian church lies a necropolis filled with ancient tombs and the ruins of previous churches.

This chain of artificial lakes was an ideal test ground for the aircraft that would carry the "bouncing bombs" used by the WWII "Dambusters" squad.

The heroic tale of a remarkable pup who refused to leave her master's side, no matter what.