The Paris Gun (German: Paris-Geschütz) was a German long-range siege gun used to bombard Paris during World War I. It was in service from March-August 1918. When it was first employed, Parisians believed they'd been bombed by a new type of high-altitude zeppelin, as neither the sound of an airplane nor a gun could be heard. It was the largest piece of artillery used during the war by barrel length if not caliber, and is considered to be a supergun.
Also called the "Kaiser Wilhelm Geschütz" ("Emperor William Gun"), it is often confused with Big Bertha, the German howitzer used against the Liège forts in 1914; indeed, the French called it by this name, as well. It's also confused with the smaller "Langer Max" (Long Max) cannons, from which it was derived; although the famous Krupp-family artillery makers produced all these guns, the resemblance ended there.
As a military weapon, the Paris Gun was not a great success: the payload was minuscule, the barrel required frequent replacement and its accuracy was only good enough for city-sized targets. However, the German objective was to build a psychological weapon to attack the morale of the Parisians, not to destroy the city itself.