This subterranean necropolis holds three floors of antler-dug grave caves.

Stone Age solar temple is aligned with the sun on each equinox and solstice.

This exclusive resort has completely covered a tiny Balkan island in a 5-star warren of guest rooms.

A tiny church on a man-made island in the Adriatic.

A curious cave church carved into the face of a cliff.

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

A luxury hotel housed in a former 15th century Gothic church.

The world's oldest Jewish library was founded by Sephardic Jews in 17th century Amsterdam.

This miniature take on the Kremlin is one Dutch man's retirement project.

This historical Netherlands fortress still bears the marks of its single siege.

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

Peek through a transparent pyramid and drink in the mountain made of books.

This small town in the Netherlands has no roads but instead, miles of canals and over 100 bridges.

Museum dedicated to "the man who moved the Earth."

A huge wooden clock that is so beautiful its creator was forcibly blinded.

Poland's oldest surviving university building holds a wealth of historic science instruments including some used by Copernicus.

The site of the first battle of World War II.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

Naturally formed rock city in Poland.

A fairytale Gothic castle steeped in Nazi treasure conspiracy theories.

Countless stories attempt to explain Poland's mysterious J-shaped trees.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

A neolithic tomb converted into a tiny chapel.

A stunning 18th-century site where pilgrims ascend by a series of punishing stairs only to return via funicular.