Apartment complexes built into huge repurposed natural gas tanks.

A once crucial portion of the Long Island railroad still lays abandoned, waiting to be turned into an urban park.

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

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Science fiction's grandfather has a fittingly marvelous tomb.

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

This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

Stone memorial that's said to predict the death of the pope and the site of the Cadaver Synod.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

This little-known relic crypt holds stolen parts of a Roman Empress.

The oldest stone house in Paris was built by its most famous alchemist.

The largest medical and anatomical collection in France lies hidden from the public.

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

How did a French journalist’s grave become the sexiest statue in the cemetery?

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

More sculpture than building, this architecturally significant temple was built on the spot of Saint Peter's crucifixion.

The middle finger of modernity.

This semi-secret storybook hedge maze is also the oldest garden in the city.

The footprint of a frustrated devil in Munich's Cathedral.

One man's personal kunstkammer, museum and shop.

King Ludwig I's collection of beautiful women.

Hidden in an Italian forest is a completely homemade amusement park that has taken a lifetime to create.

One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.