The Paris Gun
The Paris Gun was a gun that became obsolete nearly as soon as it was created. The massive gun was built in order to be able to shell Paris from behind German lines. This supergun was the largest gun employed during World War I by barrel length and it was almost unheard of for the time. When the first shells landed on Paris there were numerous theories, including the idea that the shell had come from a hidden gun somewhere in France. Other theories suggested that the shells had come from a zeppelin.
However, it did not take long for reconnaissance to locate the massive gun just over the German border. It was encased in concrete and was able to fire a shell so far that the curvature of the Earth had to be taken into account when plotting trajectory. For all the hype around the gun, it was not a very effective weapon, except in the psychological sense.
The gun was only good for targets the size of a city and it was never the sort of threat to Paris that a bombing was. The payload of the gun was small and the barrel of the gun needed to be replaced frequently. The objective of the Paris Gun was more psychological, to scare and demoralize Parisians rather than really destroy the city.
The specifics for the Paris Gun were never known as all surviving guns and the documentation regarding them was destroyed. As per the Treaty of Versailles the Germans were supposed to turn over a Paris Gun to the Allies but they never complied. The Germans largely abandoned the idea of a supergun and instead focused more on rockets, created the V2 rocket for World War II.