A mysterious structure left behind on the Polish mountainside, sometimes called "Hitler's Stonehenge".

The oldest stone house in Paris was built by its most famous alchemist.

Abandoned railway line circling the city of Paris.

France's most famous cemetery, with some of its most curious tombs.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

Enormous, fluffy dogs frolic around this museum dedicated to a legendary Saint Bernard.

Denmark's famous pseudo-hill.

Tiring of digging out the door to this church for every service, it was eventually abandoned to the sands.

Climb to the top of its spiral spire for spectacular views of Copenhagen.

A natural wonder in south Denmark.

This secret collection of the strange and occult was rediscovered in a forgotten room of an old house.

Danish tourist destination well known for open stalls where vendors sell marijuana on the street.

Created as a "model concentration camp" to hide Nazi atrocities from the international community, this former camp-ghetto now holds a museum and memorial honoring the lives lost within.

The palace window through which Protestant nobles flung two Catholic regents, thus sparking the Thirty Years' War.

A saintly relic hidden in a Prague church basement.

Two stunning 17th-century libraries and a cabinet of curiosities.

A living descendant of the famous tree that helped Isaac Newton develop the theory of gravity.

Despite dreams of constructing this fairytale mansion, Albert Gotthard Nestor Cedergren never saw its completion.

The ruins of a monastery where Saint Bridget of Sweden had her revelations recorded.

Sweden's first electric elevator remains hidden within a fortress-like building.

Sweden's southernmost prehistoric rock art is hidden just outside Gothenburg.

Gravity seems to fail at this intriguing Swedish attraction.

A memorial to a Swedish hockey player's "encounter" with aliens.

Despite the groans of archaeologists, this astounding groups of Bronze Age petroglyphs have been painted red for tourists.