This extensive collection covers thousands of years of Polish military history.

This picturesque castle in Poland has a rich history, including a legend that ties it to lost Incan treasures.

This fairytale medieval church in Poland originally spent centuries in Norway.

These massive moving sand dunes have swallowed an entire village and are still on the run.

This man-made mound monument was built out of soil from all across the Polish empire.

This uber-quaint Polish village is known for its habit of painting elaborate floral motifs on just about everything.

A fairytale Gothic castle steeped in Nazi treasure conspiracy theories.

An underground city of salt.

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

A Stockholm metro station is home to an escalator that is not only the longest in Sweden, but among the longest in the world.

Stockholm's deepest subway station is also a stunning ecological wonder.

A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

A rare surviving Victorian greenhouse designed specifically to house giant water lilies.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

The world's first combination ski slope and power plant.

Pre-Nazi swastikas adorn the sculpted pachyderms flanking this brewhouse portal.

An ultra-modern diversity park in Denmark has turned an entire city block electric pink.

Copenhagen's celebrated theme park and gardens.

These 4,444 wooden steps straight up a mountain were built to serve a hydroelectric power station but these days, they're mostly used by hikers.

Thousands of birds nest in the cliffs while arctic foxes prowl below.

Longest road tunnel in the world features an air treatment plant and the illusion of daylight.

At this Arctic outpost, get to know members of the indigenous Sámi community and meet their herds.

At the very tippity-top of Europe is a cliff from which you can view the midnight sun.