During the final years of World War II, the Nazis forced thousands of prisoners to carve out an elaborate system of tunnels under Silesia’s Owl Mountains. Almost all documentation related to the project was destroyed before the German surrender, meaning that the purpose and the true extent of these tunnels, codenamed Project “Riese” (or “Giant”), remain shrouded in mystery to this day.

The secret tunnels under Silesia have attracted numerous treasure hunters over the years, convinced that somewhere under this low mountain range lies a trove of Nazi loot. Theories about the true purpose of the Silesian tunnels range from a secret underground train system to transport stolen gold and other treasures, to the final hiding place of the Amber Room, to a covert weapons lab working on the “Wunderwaffe,” a top-secret new weapon under development by the Nazis.

Most historians now believe that the tunnels were probably an attempt to move key factories underground and away from Allied bombing raids, but this hasn’t stopped others from searching further. The full extent of the Nazi tunnel system under Silesia has yet to be uncovered, and the region continues to attract treasure hunters from all over the world, looking for new stores of Nazi gold.

The tunnels were abandoned after the war, and for several decades, only experienced cavers and treasure hunters dared to explore the many underground complexes dug out during Project Riese. Beginning in the 2000s, however, many known sections of the tunnels have been secured and opened to the public. The most famous of the Silesian tunnels are the ones under Ksiaz castle, but more than a dozen other self-contained tunnel systems have been discovered throughout the Owl Mountains, notably in the modern Polish towns of Osówka, Rzeczka/Walim, Soboń, Jawornik, and Włodarz.