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

Budapest's Communist-era statues, banished to the suburbs.

Sitting beneath an unremarkable residential building is a museum devoted to classic Pinball.

A Trabant car, a kangaroo statue, and a plethora of plants take over this abandoned-factory-turned-bar.

Museum dedicated to the terror regimes of Hungary.

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

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

A series of caves under Castle Hill that once held the prisoner Vlad Tepes, better known as Count Dracula.

Martin Bricelj Baraga's monolithic sculpture measures the blueness of the sky, and changes color to match it.

The haunting ruins of a military fortress turned astronomical observatory sit on a Sarajevo mountaintop.

Originally envisioned by King Ludwig II to rival Versailles, Herrenchiemsee's "New Palace" remains half-gilded for lack of funds.

This museum tells the story of lake dwellers from 4000 BC to 850 BC, replete with stilt houses and goats.

This poignant tribute to the White Rose movement showcases artifacts related to the heroic anti-Nazi struggle.

The telescope that discovered Neptune.

This beautiful 19th-century temple stands atop the ruins of its predecessor.

The scars of World War II are preserved on its walls.

A subterranean display of public art and architecture.

One of the most easily accessible examples of the rare 1960s UFO houses.

These bronze pamphlets remember a remarkable group of Germans who fearlessly opposed the Nazis in the very center of their power.

An abstract representation of weather in Sweden.

A concrete mushroom that divided Stockholm residents for nearly a century.

A 130-year-old urinal is the oldest of Stockholm and perhaps all of Sweden.

Several cannonballs of mysterious origins decorate two buildings in Stortoget's main square.

The library's graceful rotunda is like a bibliophile's temple.