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

This structure atop a craggy cliff in one of the most beautiful parks in Paris boasts an unbeatable view.

The narrowest street in Paris.

The first green space constructed on an elevated viaduct.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

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

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

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

Notorious artist squat renovated into legal studios.

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

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

Meant to resemble a massive tree, the building is the best example of Spanish organicist architecture in Madrid.

This postmodern apartment block was designed to be a vertical neighborhood.

One of the world's oldest universities houses an incredible collection of anatomical models, mummified body parts, and human bones.

A lush indoor jungle fills this bustling Madrid transportation hub.

Madrid's romantically ornate crystal palace has been offering shelter to plants and art for over a century.

Started as a kitchen garden, now the second largest botanical garden in the world.

This Berlin island is home to a flock of free-roaming peacocks, as well as a historic ivory folly.

A collection of abandoned typographic signs from Berlin and beyond.

This conical hill was built as a testing ground for an engineer known as the Glider King.

These former Nazi anti-aircraft towers offer a singular view of the north of Berlin in the summer, and a sanctuary for bats in winter.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

A stairway to heaven, or to nowhere at all.