The hollow trunk of this 650-year-old tree was used as a hideout by two Jewish brothers during World War II.

A monument to Poland's famous alleged UFO abduction case.

A marker honors the brief peace found by a tormented artist in the fictional land of Kcymaerxthaere.

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

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

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

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

Modern art commemorates one of the first military actions of World War II.

Memorial to the lost dead of Gdansk, whose resting places were destroyed during World War II.

The site of the first battle of World War II.

Deep in a Polish forest, this collection of Iron Age megaliths still holds onto many of its ancient secrets.

The original stone marking the interwar border between Germany and Poland sits in this unassuming village park.

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

This ex-prison dates back to the Middle Ages.

A collection of old neon signs from all over Poland line a secluded courtyard.

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

Polish luminaries and masterful folk art find sanctuary in this unique cemetery.

An early creation by Erich Mendelsohn.

A program of Communist glamour in post-Stalin Poland is preserved in this colorful museum.

An underground city of salt.

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

A peculiar family mausoleum built by a German nobleman captivated by the occult.