The order had gone out months earlier, and now the great gun sat at the end of its long special spur line, sixteen kilometres north of the target. It was the largest artillery piece ever built, and the men of Heavy Artillery Unit E-672 were about to use it in combat for the first time. Sevastopol lay in ruins before them. For days, German artillery of every calibre had pounded the Soviet city, and soon the enormous weapon would add its own voice to the bombardment.

The gun could not simply be pointed and fired. It stood on a specially designed chassis supported by eight bogies running on two parallel railway tracks. To aim at each Soviet fort in turn, the Germans had laid four semicircular tracks at the end of the spur, so the entire immense structure could traverse across the arc and bring its barrel to bear on one strongpoint after another. Further outer tracks had been laid for the huge cranes needed to assemble the machine.

Heavy Gustav had arrived in early March 1942, a gun train of twenty-five carriages stretching a kilometre and a half, almost a mile, behind it. Assembling and moving it into position took four thousand German troops five weeks to complete. Five hundred men formed the gun crew itself. By the 5th of June 1942, everything was ready.

The gun that waited to fire that day was the product of a plan that had its roots in the First World War. During that earlier conflict, the Germans had built and used several large railway guns, including the infamous Paris Gun. But what marked out this new weapon was its sheer size, far beyond anything seen before. The German High Command had asked the armaments firm Krupp to design a gun that could knock out the forts of the Maginot Line in France in the event of another war.

Crucially, the gun had to be able to shell those forts while remaining out of range of French artillery. Krupp’s answer was a calibre of eighty centimetres, or thirty-one and a half inches. To put that in perspective, the Bismarck, Germany’s future largest battleship, mounted guns of thirty-eight centimetres, fifteen inches. Heavy Gustav would be more than twice the size of the largest naval guns of the period.

Its barrel stretched thirty-two and a half metres, a hundred and six feet long. Throughout the 1930s and into the early 1940s, Krupp’s designers and engineers worked to create a functioning gun of this enormous calibre. The first test firing came on the 10th of September 1941. A series of further tests followed, with the gun flinging a seven-ton armour-piercing shell 37,210 metres, twenty-three miles.

Two guns were built. Krupp charged the German government seven million Reichsmarks for the first, a sum roughly equivalent to thirty million dollars today. The second, named Dora, cost the same again. The first weapon was given its own name, Heavy Gustav, after the Krupp family.

Now, on the morning of the 5th of June 1942, Heavy Gustav opened fire on the forts ringing Sevastopol. The Soviet city was defended by a ring of strongpoints, and the gun would be used to reduce them one by one. The firing rate was slow, by any measure. Heavy Gustav could fire one seven-ton armour-piercing shell, or one five-ton high-explosive shell, every thirty to forty-five minutes.

In combat it never actually reached even this rate of fire. But the effect of each round was devastating. On the 6th of June, the gun was turned on the Ammunition Mountain, an undersea ammunition magazine in Sevastopol’s Northern Bay. The extraordinary store was thirty metres underwater, protected by ten metres of concrete.

Nine rounds of seven-ton armour-piercing shells from Heavy Gustav obliterated the magazine. The blast also sank a Soviet warship in the bay. The giant coastal gun batteries inside the Maxim Gorky fortresses were next. On the 17th of September, five shells were fired at the positions, knocking out Maxim Gorky I and damaging its sister fortress, Maxim Gorky II.

From the 5th to the 17th of June, Heavy Gustav targeted forts and ammunition magazines, causing immense damage across the besieged city. In that period the gun fired forty-eight rounds, equating to 336 tons of explosives. It was a small portion of the 30,000 tons of munitions that German artillery of all calibres dropped on Sevastopol, but the damage wrought by this one gun was impressive out of all proportion to its share of the tonnage. The bombardment of Sevastopol wore out the gun’s barrel, and it was replaced.

The next stop for Heavy Gustav was Leningrad, another Soviet city under siege. There, the gun had just become fully operational when a proposed attack was cancelled. Heavy Gustav was placed in storage for the winter. Its sister gun, Dora, was deployed close to Stalingrad but was withdrawn to avoid the Soviet encirclement of the city.

Neither gun saw any further action. They were so immense, so difficult to move, and such obvious targets for enemy aircraft that they could not be used again on a battlefield where the Germans no longer held command of the skies. Heavy Gustav was blown up on the 14th of April 1945 to prevent its capture by the US Army. Its ruins were discovered fifty kilometres, just over thirty miles, southwest of Hemmets in a forest.

The area fell under Red Army control, and in the autumn of 1945 Soviet specialists studied the wreckage. The gun was then sent to Meza Beg and lost to history. Dora was blown up on the 19th of April 1945. What was left was scrapped in the 1950s in West Germany.

But that was not quite the end of the story. The Germans had planned an even more terrifying version of Heavy Gustav. It was called Langer Gustav. This gun was actually smaller in calibre, fifty-two centimetres, twenty and a half inches.

The plan was to set it at Calais in France and use it to target London. Instead of firing normal shells, Langer Gustav would use long-range rocket projectiles weighing 680 kilograms, with an immense range of 190 kilometres, 118 miles. Such a weapon could have replayed the earlier Blitz, as high-explosive shells rained down on the British capital every few minutes. Fortunately for London, this nightmare was stopped by the Royal Air Force.

Bombers hit the gun plant at Essen several times, and the one example under construction was damaged and never finished. It can be argued that Heavy Gustav and Dora were a waste of resources in a war where such massive creations could so easily fall victim to air attack. They fitted Hitler’s demands for ever bigger machines of war, but in the final analysis, their impact on the battlefield was slight, and they consumed huge amounts of precious resources to achieve it.

The largest artillery pieces ever built fired their last rounds years before the war ended, and ended their lives as explosives, scrap, and ruin.