A 19th-century artistocrat's mansion, turned into a library, hidden in a modern library.

A museum dedicated to a secret military hospital hidden beneath a castle in Budapest.

A two-way chairlift system between Zugliget and the highest point in the city.

This Hungarian castle was built out of cardboard and wood until it proved so popular that it got upgraded to stone.

This Budapest bar allows drinkers to leave their mark on the walls, floor, and even ceiling.

A museum in the childhood house of a medical pioneer.

Built in 1870 at the bridge that connected Buda and Pest, this spectacular incline railway was almost lost forever during World War II.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

The world's longest and deepest train tunnel.

Traversing steep Alpine slopes, this engineering marvel is one of the longest and highest funiculars in the world.

Incredible labyrinth of man made caves.

Two gravestones clasp hands over the religious divide.

Now a museum, this building was once a 14th-century monastery and the place where the founder of the Dutch nation lived and was assassinated.

This enormous storm barrier is one of the largest moving structures on Earth, with each arm taller than the Washington Monument.

A journey through 430 years of bulbiculture in the Netherlands.

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

A market built atop boats is one of the few floating holiday markets in Europe.

Science meets wall art to highlight the beauty of famous mathematical equations.

A tiny house showcases the busy life of a local gnome.

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

Towering sandstone rock formations where 'The Chronicles of Narnia' was filmed.

A museum of alchemy and a basement laboratory all housed in an ancient building in a Gothic tower.

This unusually shaped mountaintop tower looks like a UFO floating over the city.

Bohemian paradise covered in majestic forests.