Gustav Gun

The "Gustav Gun" was built in Essen, Germany in 1941. The strategic weapon of its day, the Gustav Gun was built at the direct order of Adolf Hitler for the express purpose of crushing Maginot Line forts protecting the French frontier. To accomplish this, Krupp designed a giant railway gun weighing 1344 tons with a bore diameter of 800 mm (31.5") and served by a 500 man crew commanded by a major-general.

Here's some video of the Gustav gun.