The Scwerer Gustav is classified as a Supercannon, measuring at a whoping 800mm Calibur, it could only be transported on a double set of rail tracks (also making it a railway gun).
It could punch through 15 meters of Concrete and 5 meters of solid metal armor. Unfortunately it was destroyed near the end of the war to prevent capture. One of its' shells is on display in a Polish war museam.
A second gun "Dora" was also built but dismantled, and a third was planned by never started.