Under a hill in Moselle, near the town of Thionville, lies a massive underground labyrinth known as the Hackenberg Fort. This structure is one of the largest components of the Maginot Line, a series of defensive fortifications built by France in the 1930s to protect against potential invasions from Germany. The fort spans over ten kilometers of tunnels and galleries across 160 hectares, connecting nineteen separate combat blocks. Originally constructed between 1929 and 1936 by around 1,800 workers, the fort was equipped with barracks, a kitchen, a telephone exchange, and an autonomous power plant. Today, a restored electric train from the 1930s allows visitors to tour the underground complex, now transformed into a museum open to the public. The construction of the Hackenberg Fort was a response to the devastation France suffered during World War I. The country sought to build a formidable defensive line along its eastern border to prevent future invasions. The fort was officially handed over to the French army in 1936. Workers used basic tools and early machinery to carve the structure into the Moselle rock, creating an underground city complete with storage for M1 ammunition, medical facilities, and an electricity plant. The design aimed to provide both shelter and operational capacity in the event of war. Despite its strategic position, the Hackenberg Fort remained unscathed during the German invasion in June 1940. However, it was later occupied by German forces, who repurposed it during much of World War II. In 1943, the Germans set up a factory within the fort that employed deportees and Soviet prisoners under harsh conditions. Two explosions occurred in the galleries during this time, causing damage to the structure. The fort regained military significance in November 1944 when American forces under General Patton clashed with German troops in the area. After World War II, as tensions shifted to the Cold War, the Hackenberg Fort was repaired and gradually transitioned from a military installation to a historical site. Today, it draws history enthusiasts and families looking for a unique experience. The fort offers a rare look into an era when France relied on concrete and steel to defend itself. It stands as a tribute to the engineering of its time, the human stories behind its construction, and the transformation of military history into a public educational resource.