This abandoned but well-preserved Prussian fortress sits abandoned in a Polish forest near the German border.

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

An extensive network of secret tunnels was created for the Nazi project "Riese."

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 extensive collection covers thousands of years of Polish military history.

The site of one of the largest prison escapes of World War II.

This open-air museum was originally a World War II testing ground for German missiles and rockets.

One of the largest amphitheaters in Europe is a former Nazi gathering place that still stands today.

This 13th century Gothic brick castle is the largest in the world by surface area.

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

An abandoned Nazi explosives plant founded by the inventor of dynamite is now an underground museum.

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

A tribute to the Polish insurgents who fought in the failed attempt to end their city's Nazi occupation.

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

A fairytale Gothic castle steeped in Nazi treasure conspiracy theories.

The site of the first battle of World War II.

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

A remnant of the original World Trade Center complex continues to serve as a commuter gateway.