Hideout of thieves, monks, and goat-footed gods.

Nationwide network of co-ownership underground catastrophe shelters.

The underground sanctuary of a mysterious ancient cult.

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

Yugoslavia's biggest underground airport lies in ruins.

An opal mining town in the middle of Australia where people live, worship, and shop underground.

Caves famous for the dragon-slaying saint who lived there.

An abandoned Jazz Age dream home and con man's hideout in Death Valley.

Russia's own leaning tower was built on a tragic historical fable.

Stalin's secret bunker.

Cold storage for agricultural biodiversity.

The deepest hole drilled in the name of science, where evidence of Precambrian life was found.

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

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

The largest of London's subterranean rivers and once a mad, bad center of London life.

One of the largest, oldest, darkest, quietest motel rooms in the world.

This ancient subterranean megalopolis is known for the hot oil traps that were installed above each of its entrances.

A small but immensely extravagant palace built by the "Night King."

This bridge provides safe passage for squirrels.

A miniature Italianate fantasy village on the coast of Wales and the set of ITC classic "The Prisoner."

Step inside Lucy, the world's largest "elephant."

One man's vision and a labor of love of many.

Once the gateway to the utopian dream-village of a Civil War embalmer.

A bizarre collection of gifts to North Korean dictator Kim Jong Il from the despots of the world.