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

The decaying ruins of Wales' first asylum for the mentally ill.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

The curious underground history of keeping Paris clean.

19th century tunnels burrowed under Liverpool.

WWII rocket launchpad in rural Germany and site of operation backfire.

Climb the dead electric pylons of Messina.

The metal-free wooden towers of Brück.

A restaurant and observation deck built in a unique broadcasting antenna.

Gliwice Radio Tower, likely the tallest wooden structure in the world.

A church of bones, decorated with 40,000 human skeletons.

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

A spectacular cave and a beguiling lake.

The fairy tale castle of the "fairy-tale king."

WWII artifacts, including the building itself.

Hiding the longest subterranean river known in the world.

A sunken idol welcomes parishioners to Davy Jones' Locker.

Venice's "Island of the Mad."

This hospital once pioneered the use of electroencephalograms, but fell into maltreatment, abuse, and eventual abandonment.

A nightly tradition fills the streets of Flogsta with a collective scream.

An underground city of salt.

This attraction was abandoned after the Chernobyl disaster.

Now a pile of ruins and rubble, this was a lovely mountain town as recently as 1960.

Infamous and now demolished panopticon prison built by its former inhabitants.