Once home to some 250,000 volumes and priceless art artifacts, this Polish estate was a target of bombing during World War II.

An unusual double-stelae obelisk with ancient and mysterious origins.

One man’s collection of photography books has transformed into a charming exhibition space.

The ruins of a medieval monastery built on a perilous slope.

An open-air museum of 18th-century homes and churches transported from various villages in Southern Poland.

Relive a Polish uprising via an enormous, century-old cyclorama.

The traditional Polish croissant, folded 81 times into the shape of a horseshoe, is a legally protected cultural treat.

The world’s narrowest house makes for an awkward, four-foot-wide living space.

This sprawling concrete suburb of Kraków was designed by the Soviet Union as the model proletarian city.

This steel frame is a skeletal mirage of a lost 16th-century stone building.

Home to the world's oldest functioning orrery.

The world's first floating farm is staffed by one human, three robots, and 35 cows.

Visit the ancient, mysterious Roman catacombs ... in the Netherlands.

Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

Sail in for a ’dam good slice.

These 18th-century paintings of Warsaw were used to reconstruct the city after World War II.

Small church on top of a huge rock column.

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

A line of disused military bunkers built to defend against Nazi invasion lies hidden on the outskirts of the city.

A unique hotel for the artistic soul.

This 3,800-year-old site contains markers used to study the movement of the sun and moon and create a lunar calendar.

It's unclear what caused the fire that led to this opulent hotel's demise.

Space-age monument to an early 20th-century uprising.

Designed as a reminder of one of the darkest moments in Austrian history.