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

The art museum is decorated with creatively remodeled pre-Nazi swastikas.

The old observatory in Copenhagen has a spiral corridor used as a racetrack.

The only surviving piece of Copenhagen's 16th-century defensive wall.

This pair of gilded girls atop an Art Deco building in Copenhagen were built to function as a weather indicator.

This contemplative sculpture made out of old machine parts is meant to be the first in a series of sculptures around the world.

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

The de facto symbol of Copenhagen is this bronze fairytale which cannot seem to keep its head for very long.

Copenhagen's celebrated theme park and gardens.

A statue commemorating a Polish dog who took his role as man's best friend very seriously.

At first glance, this area seems like just another city park, but the lush landscape hides a dark past.

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

Incredible labyrinth of man made caves.

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

Underground attraction offering tours of the foundational ruins of a church nave that collapsed in 1674.

The cutest way to stay safe in traffic.

The world's largest bicycle shelter can hold more than 12,500 two-wheelers.

'The Letters of Utrecht' lays out a ribbon of poetry one letter at a time, with no final verse in sight.

The block of aged dairy is probably still edible after 2,300 years in a peat bog.

A tiny microvalley etched into the side of Knocknarea Mountain hides another world.

A meandering boardwalk across the bogland leads to a "stairway to heaven" with sublime views.

A colorful narrow alley in Cuba is full of street art and sculptures made of found objects.

The 20-mile causeway that separates the Netherlands from the sea.

The history of sea-faring refuse and disaster is on cramped display in this Netherlands folk museum.