Under the hills of the Boulonnais, near the English Channel in northern France, Nazi Germany built an underground complex in the Pas-de-Calais region during World War II. This facility was intended to house the V-3 cannon, a powerful weapon designed to fire shells repeatedly at London from a distance of 165 kilometers. The project started in 1943 and involved 430 German miners and nearly 5,000 Russian prisoners of war who were forced to work under harsh conditions. Unlike the earlier V-1 and V-2 weapons, which used guided missiles and rockets, the V-3 relied on a system of long, multi-charge cannons with tubes up to 105 meters in length. Originally, plans called for two installations with 50 cannons each, but due to shortages of materials and resources, only one installation with 25 tubes was completed. Allied intelligence had been monitoring the area and detected unusual activity around the site as early as September 18, 1943. This led to an initial bombing on November 5, 1943, but the attack failed to stop construction because the underground structures were protected by thick concrete. The real concern for the Allies was the V-3's potential to deliver continuous fire on London, which would have made it a major threat. To prevent this, on July 6, 1944, the 617th squadron of the Royal Air Force conducted a raid using Tallboy bombs, specially designed to penetrate deep into the ground. These bombs successfully destroyed the site before it could become operational, collapsing several underground galleries and burying months of work. The V-3 cannon never fired a single shell at London, and the site was later abandoned by the Germans. In 1945, the Allies dynamited the remaining structures to ensure the site could not be used again. Today, remnants of the complex remain as a historical memorial, offering a glimpse into the engineering efforts and the strategic concerns of World War II. The site stands as a reminder of the technological ambitions and the destructive potential of wartime innovation.