The Międzyrzecz Fortification Region — known by its German acronym MRU, short for Festungsfront im Oder-Warthe-Bogen — is an enormous network of subterranean tunnels and bunkers stretching beneath the forests of western Poland. Gates describes it on camera as "80 miles, that's eight-zero miles, of subterranean tunnels connecting 100 bunkers," calling it "nothing short of an underground city" in its day. Construction is believed to have begun as early as 1934, when the area was still part of Germany, and in 1943 Hitler reportedly moved vital wartime operations into the complex as Allied bombing intensified. At the war's end, many sections were imploded or deliberately sealed, leaving large portions of the network unexplored for decades. Today the tunnels are also known as a major hibernation site for European bats, drawing naturalists alongside history enthusiasts. Gates came here chasing a specific wartime lead: whether Daimler-Benz relocated an aircraft engine factory into the MRU, potentially connected to a Nazi long-range bomber program targeting America.
Construction of the MRU fortification and tunnel network begins, in territory then part of Germany.
Hitler reportedly moves vital war operations underground into the MRU as Allied bombing of the region intensifies.
At the end of World War II, many tunnel sections are imploded or sealed; the area passes to Polish control.
Gates and researcher Pawel Pietkiewicz investigate a previously sealed section of the MRU in Expedition Unknown S15E01, 'Hitler's Amerikabomber.'
Researcher Pawel Pietkiewicz, described by Gates on camera as one of the world's foremost experts on the MRU tunnel system, serves as the primary guide and source during the S15E01 investigation. Pietkiewicz and translator Oliver Kruger present Gates with what they describe as a recently declassified plan of the complex — a document purportedly depicting a sealed section of the tunnels unseen since the end of World War II. Pietkiewicz tells Gates directly that documentary evidence points to Daimler-Benz moving an aircraft engine production facility underground into the MRU network in 1943 and 1944, with records suggesting as many as 1,500 machines were operating there.
Mainstream historical understanding holds that the MRU was one of the most ambitious fortification projects undertaken in interwar and wartime Germany, representing a significant investment in defensive and logistical infrastructure. The network's estimated 80 miles of tunnels and roughly 100 connecting bunkers made it a logical candidate for sheltering sensitive industrial and military operations as the war turned against Germany after 1942. Historians have long documented the Nazi practice of moving arms production underground to shield it from Allied air raids, though the specific details of what was manufactured at individual sites often remain contested or incompletely documented.
The episode's central question — whether the MRU hosted production connected to a German long-range bomber capable of reaching the United States, sometimes called the Amerikabomber — sits in genuinely debated territory. Historical records confirm that several German aircraft manufacturers, including Daimler-Benz, explored long-range bomber concepts during the war, but whether any such program reached advanced production stages, and whether any evidence of it survives in the MRU, remains an open question that the episode investigates rather than resolves.
What the S15E01 episode contributes most concretely is the documentation of a previously sealed chamber and its contents — including wartime documents and a February 1945 order — and the application of 3D laser scanning technology to newly accessed sections of the complex. Whether these findings advance the specific Amerikabomber hypothesis is left for ongoing research; Gates' role here, as on most Expedition Unknown investigations, is to surface evidence and put it in front of experts, not to deliver a verdict.
Gates describes the MRU on camera as '80 miles, that's eight-zero miles, of subterranean tunnels' — a scale he calls 'nothing short of an underground city.'
Construction of the MRU network is believed to have begun as early as 1934, at a time when the region was still part of Germany rather than Poland.
The MRU tunnels are today considered one of Europe's most significant bat hibernation sites, sheltering large colonies during the winter months.
Researcher Pawel Pietkiewicz, Gates' guide during the S15E01 investigation, previously worked with Gates on a separate, heavily flooded section of the MRU where the team reportedly found evidence of Nazi-looted artwork.
The MRU tunnel system is generally accessible to visitors, with guided tours available through portions of the network near the town of Międzyrzecz in western Poland. Visitors should expect a cool, damp underground environment and are advised to wear warm layers and sturdy footwear; check current local advisories and tour operator schedules before visiting, as access to specific sections may be restricted for conservation or safety reasons.
Zielona Góra, approximately 40–50 kilometers to the south; the town of Międzyrzecz itself is the closest settlement to the tunnel entrances.
Late spring through early autumn is generally the most comfortable period for visiting, with milder surface temperatures and easier access through the surrounding forest trails. Winter visits are possible but conditions can be cold and wet.
Wrocław (Breslau)
Wrocław (Breslau) is a major Polish city with deep World War II history investigated by Gates, sharing the same regional context of Nazi-era fortifications and wartime secrets in western Poland.
Berlin
Berlin sits at the heart of the Nazi command structure that built and operated the MRU, making it a natural companion site for understanding the political and military decisions behind the tunnel complex.
Poland
The Poland database entry likely covers overlapping wartime investigation content, connecting Gates' broader exploration of Nazi-era sites across the country.
Normandy Beaches, France
Normandy Beaches, France represents the Allied military pressure that accelerated Nazi underground construction programs like the MRU, offering direct thematic connection as Gates explores both sides of the World War II conflict.