This abandoned dream castle and its cemetery look out over the city that once inspired their creation.

This outdoor exhibit features a historic wooden dam once used to help boats navigate the Ohio River.

A mud statue stands in the dark tunnels, seeming to hold up the weight of the city above.

The history of radiation is on safe display at the foot of a radiation science laboratory.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

A mechanized clock where a man fights off a dragon, crab, and rooster.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.

A haunting collection of the vanished and disappearing natural world.

Dedicated to the art of hunting, with juxtapositions of contemporary art.

The approximate spot where the last Grand Master of the Knights Templar was put to death.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

Notorious artist squat renovated into legal studios.

A piece of stonework relief commemorates the site where the Devil's rage, and odd apparel, destroyed a house.

The oldest cemetery in Belgium.

The spectacular and eerie moss-covered grave site of 19th-century Belgian merchant.

You can spot this teeny tiny stained-glass window from the Bonifacius bridge.

The bronzed skull of a beheaded traitor was affixed to the city gate in the 17th century.

Christ's blood preserved in a Belgian town.

One of Jeff Koons’s largest sculptures and his first commemorative work.

If you look closely at the oldest bridge in Paris, you might notice hundreds of ugly faces leering from its sides.

A magical, macabre wax museum, beloved by locals, uses real artifacts to depict an assassination from the French Revolution.

The industrial architecture honors the laborers who brought the Exposition Universelle of 1900 to life.

The floorboard scrawling of a tragic madman is on public display outside a Paris hospital, much to the dismay of its opponents.