What was once an underground Nazi city is now Europe's largest bat reserve.

Built to transport German U-boats through the never-completed Masurian Canal, the locks near this Polish village are now home to a rope course.

This hilltop ruin is of the hundreds of stone columns built in tribute to the first German chancellor.

This lakeside bunker complex in Mamerki, Poland, housed the Supreme Military Command of Nazi Germany during World War II.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

A 90-ton, gravity-defiant steam locomotive shoots for the skies in this bizarrely beautiful public artwork.

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

Warsaw's fantastically preserved gasworks...almost.

The vertical-axis windmills of ancient Persia have been grinding flour for 1,000 years and counting.

A Jenga-style cylindrical tower of books that appears to go on forever.

Mirrors and glass shards cover every inch of this arrestingly beautiful mosque.

Hydraulic systems in an ancient underground Iranian city.

This 17th-century structure is a beautiful testimony to traditional Persian architecture

This mosque turned crypt is an amazing display of Persian-Islamic architecture.

Iran's colorful terraces are a rare geological masterpiece.