This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

Beyond being incredibly fun, not much can be said about the Hole with certitude.

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

A well-preserved network of cavernous tunnels have spent silent centuries removing Toronto's waste.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

Beautiful carved tunnels.

A tunnel dug through the side of a mountain by hand provides convenient yet death-defying access to a small village in China.

The oldest of Venice's plague quarantine stations, designed to keep the medieval city safe from disease.

Venice's "Island of the Mad."

Fountain where visitors can earn a license to be mad.

A 16th-century horror show built in a lovely Italian garden.

Jesus' footprints and the first catacombs in the world.

An alchemist's "magic door" stands in the middle of a Roman park.

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

This sword isn't waiting for a king to pull it out. It was placed there by a saint.

One of the oldest masonic museums in Paris.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

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

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

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

David Sedaris called it "The greatest shop in the world."

Hundreds of hidden passageways weave through the old quarters of Lyon.

One of the greatest maze designer’s greatest mazes.