That is the Landkreuzer P. 1000 Ratte, mounted largest artillery gun in the world (800mm) it fired a projectile 3/4th's the size of a T34 tank.
nope, thats the P. 1500 Monster, but the facts are correct.
The P. 1000 Ratte is "just" a giant tank with two 280mm battleship guns, five times heavier then the "Maus".