42 cm Gamma Mörser in the context of List of the largest cannon by caliber

Fort Douaumont (French: Fort de Douaumont, pronounced [fɔʁ də dwomɔ̃]) was the largest and highest fort on the ring of 19 large defensive works which had protected the city of Verdun, France, since the 1890s. By 1915, the French General Staff had concluded that even the best-protected forts of Verdun could not withstand bombardments from the German 420 mm (16.5 in) Gamma guns. These new super-heavy howitzers had easily taken several large Belgian forts out of action in August 1914. Fort Douaumont and other Verdun forts were judged ineffective and had been partly disarmed and left virtually undefended since 1915. On 25 February 1916, Fort Douaumont was entered and occupied without a fight by a small German raiding party comprising only 19 officers and 79 men, entering via an open window by the moat. The easy fall of Fort Douaumont, only three days after the beginning of the Battle of Verdun, shocked the French Army. It set the stage for the rest of a battle which lasted nine months, at enormous human cost. Douaumont was finally recaptured by three infantry divisions of the Second Army, during the First Offensive Battle of Verdun on 24 October 1916. This event brought closure to the battle in 1916.