The underground quarry of St-Leu-d’Esserent is located in the department of Oise and originally opened for the extraction of limestone for building purposes. By the end of 1939, the quarry had been developed by the French as an underground factory for the manufacturing of aircraft fuselages, specifically that of the LEO 45 bomber. The company Brissoneau & Lotz, who were contracted to produce the fuselages, developed the underground workings by concreting 3000m² of galleries, and installing heating, ventilation and accommodation facilities for the 1100 strong workforce.
With the invasion of France in 1940 and the subsequent arrival of the occupation troops in June 1940, the production of aircraft underground ceased. The Wehrmacht seized the underground facility on behalf of the Luftwaffe for their use as a production and storage complex. In 1943, the Todt Organisation were tasked with upgrading the factory to allow for the storage and assembly of the V1 bomb; one of three such sites in France. They constructed new access roads, defensive blockhouses, armoured blast doors, protected ventilation shafts and concrete shelters for the Flak air defences. Further work was undertaken to the interior by concreting a further 10,000m² of tunnels, forming more than 700 storage and production areas that eventually covered 10 hectares. To support the underground workings, they installed generators and a ventilation system; they also constructed a communications centre, specialised workshops, offices, barracks, infirmaries and kitchens for the workers. French workmen who were employed by the Nazi’s to undertake the initial work with Russian prisoners of war taking over once Service du travail obligatoire (forced labour sent to Germany) was initiated. The site was named Leopold Low 1106 Feldmulag (Leopold Field Munitions Camp/Depot) and became one of their most important assembly and storage facilities for V1 (Vergeltungswaffe) flying bombs. Strategically located near to the rail yard by the River Oise, the underground factory was well protected by a thick ceiling measuring up to 30m thick. In early 1944, the first train transporting V-1 flying bombs arrived from the Nordhausen factory in Germany.
Shortly after the launch of the first V-1 flying bombs against London, Leopold 1106 was bombed by the Allies. Three massive bombings were launched on 4 July, 7 July and 7 August, 1944. The last of these was carried out with 6-ton “Tall Boy” bombs, which effectively stopped the activities in the underground factory. Before the RAF raids, over 80% of all V1s were assembled at St Leu. The weight and number of attacks on the site and the strong resistance encountered reflect the importance of the target; Bomber Command lost 45 planes and crews. Although the RAF believed that St-Leu’s-d’Esserent’s facilities had been totally destroyed, in fact only the upper level was affected. However, access to the underground factory was blocked, burying a large number of V1 and the access road and the railway line had been seriously damaged. More information can on the bombing raids can be found here.
On August 11th, 1944, the Germans begun to retreat and were ordered to evacuate the V1 stocks and then destroy the facilities in order to prevent the Allies from accessing their weapons technology. They were only partly successful and when the US troops arrived on August 30, and engineers quickly began to study over one thousand V1 rockets that had escaped destruction. In 1946, the US Forces authorised the French army to take possession of the quarry, who then cleared out the remaining equipment. Finally, in 1947 stone extraction resumed to aid the reconstruction of the country. During the post war period, the tunnels were also used for mushroom growing, closing down at an unknown time. The tunnels now lay derelict and forgotten.