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

This tri-part waterfall is a little bit of fairytale beauty in the heart of Luxembourg's Little Switzerland.

Visit the ancient, mysterious Roman catacombs ... in the Netherlands.

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

A beautiful maze marks where three countries meet.

Incredible labyrinth of man made caves.

Home to the world's oldest functioning orrery.

Live out your childhood construction fantasies in this one-of-a-kind gyrating hotel.

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

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

This impressive collection of rare books on ancient mysticism inspired author Dan Brown.

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

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

The ruins of a medieval castle that once belonged to one of the most powerful families in the Netherlands.

Oldest Museum of the Netherlands is a true cabinet of curiosities.

A 'living museum' in Leiden, the oldest of its kind in the Netherlands.

This public library and its impressive collection of academic books have not changed since the 17th century.

A true Valhalla for cactus and succulent lovers.

This medieval castle was painted pink as part of a now-abandoned children's theme park.

A dream-like maze full of strange art, surreal spaces, and fantasy landscapes to get lost in.

A late-medieval Dutch castle with trick fountains in its whimsical water gardens.

A Dutch museum preserves the world's most diverse collection of vertical slices of soil.

The place where houses are born.