A rare example of World War I trenches from the British front.

One of the deepest indoor swimming pools in the world.

This abandoned Belgian fortress is purposefully being left to nature.

Hiding the longest subterranean river known in the world.

A magical and peaceful ruined Cistercian abbey in what is now southern Belgium.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

Each spring, a dreamlike carpet of bluebells overtakes the forest floor.

This imposing tower named after the mythical sword of King Arthur is considered the tallest freestanding climbing wall in the world.

This historical Netherlands fortress still bears the marks of its single siege.

Incredible labyrinth of man made caves.

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

This beautifully designed star-shaped fort was the premier defense system of the late 1500s.

Birthplace of the "Cat in the Cradle" and a cluster of iconic, original windmills that let the Dutch tame the seas.

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

Unsurprisingly, Holland is home to one of the most magnificent flower gardens in the world.

The Bloemen Route, or “Flower Route”, stretches 25 miles through prime Dutch tulip country as a result of “tulip mania,” the world’s first economic bubble.

A beautiful maze marks where three countries meet.

This 22-floor tower's Skydeck offers panoramic views of Amsterdam.

Peek through a transparent pyramid and drink in the mountain made of books.

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

Nestled within a German forest is an intriguing, growing rock.

A top-secret bunker now turned museum, is an eerie look at what life would have looked like in the event of nuclear war.

A surprisingly detailed model of one of the most powerful telescopes in the world.

A modern museum dedicated to the fairytales and dictionary work of the Brothers Grimm.