This is the Maus tank, it is the heaviest tank ever created (That actually moved), Its armor is so thick that no modern weapon could pierce its frontal armor, and most couldn't even get through its side armor. it only goes 8.1 MPH. The only American weapon created that could get through the Maus is a 155mm gun. Neither the Maus or the 155 ever saw action on both sides.

More information on the worlds heaviest tank
http://en.wikipedia.org/wiki/Maus_tank(i have yet to finish this tank)