A marvel of Byzantine engineering under the streets of Istanbul.

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

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

Baroque celebration of the end of the Great Plague of Vienna.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

The final resting place for the remains of 143 Habsburg royalty.

Op-art architecture designed for happiness.

Apartment complexes built into huge repurposed natural gas tanks.

The mummified right fist of a 1,000-year-old saint-king sits inside an ornate golden reliquary.

A countryside Romanian cemetery filled with colorful crosses bearing lyrical, ironic, or satirical rhymes.

Berlin's old airport.

Cold war icon and onetime "hole" in the iron curtain.

The first hand-illuminated, hand-written bible since the invention of the printing press.

A giant portrait of an ancient king looms over the river.

17th century palace once a meeting place for politicians.

Romania's triumphal arch looks out over Bucharest.