The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

This centuries-old gothic monster has been converted into a sprawling, glossy bookstore.

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

Hidden within a Netherlands crater is a stone bed that lets those who lie in it see the sky as a curved surface.

Colorful and chaotic stacked hotel in North Holland.

The attic of a four-century-old canal house hides a clandestine church.

This small town in the Netherlands has no roads but instead, miles of canals and over 100 bridges.

Established in 1638 to battle the Black Death, Hortus Botanicus Amsterdam is a treasure trove of rare flora.

Cat art as far as the eye can see.

An Amsterdam street lined with antique shops each one dedicated to a different obscure collection.

The world's only museum devoted to the wonders of fluorescence.

A garden of flying flowers.

This impressive collection of skulls and bones was gathered by an ex-zookeeper who took advantage of a pig plague.

Ancient man-made caves underneath small town Belgium.

The vast chamber inside this abandoned cooling tower looks like something out of a sci-fi dystopia.

Abandoned since 1998, this train station has become an oasis for urban explorers.

This old, abandoned hotel offers magnificent views.

An early 20th-century sanatorium, long abandoned and now being reclaimed by nature.

This collection of lurid religious statues tell the tales of a saint who stood down the devil.

This charming, hidden guild museum holds some 700 years of crossbow history.

Head underground and take a tour of the city's working sewer system, and see the underground river Senne, which defined Brussels for centuries before being covered in the 19th century.

This small window display is dedicated to the noble carrot.

A Cold War command center is hidden under an idyllic countryside.

A once-magnificent collegiate church is falling into ruin.