Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

Children's hospital in London that operates on royalties received from Peter Pan, which J.M. Barrie signed over in 1929.

A cluttered and astounding collection of antiquities and curiosities.

A Belgian self-built tower inspired by the Bible and ancient civilizations.

This sprawling Belgian fort was once thought of as impenetrable but now it is simply an abandoned relic.

No need to strap in for a "ride" on this walkable roller coaster.

The Bauhaus-style former industrial complex is considered the world's most beautiful coal mine.

Hop aboard a charming bog train to explore this rare open-air museum of peat.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

Two small chapels housed inside an ancient tree.

This anatomical sculpture raises awareness about violence against women and promotes gender equality.

Bordered by forest on three sides and the ocean on the other, the Great Dune of Pilat is the highest sand dune in Europe.

This Bordeaux parking garage has 712 indoor spaces, and one outdoor.

The beautiful bas-relief lines a sliver of a 19th-century walking path between two charming French villages.

An abandoned nightclub has become a vibrant work of art.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

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

A Gothic cathedral created from volcanic rocks.

Invisible bridge parts the waters, invoking biblical images and preserving the ominous feel of an ancient moat.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

A small world of mechanical magic and metal merry-go-rounds in a Outsider art Museum.

This natural French spring has been turned into a magical grotto by Roman and medieval constructions.

This French chateau is home to the world’s largest private collection of fighter jets.

For over 300 years this little carving has been the city’s good luck charm.