Also known as the Panzerkampfwagen VIII Maus. It was one of the few super-heavy tanks that ever got off the drawing board.
The Maus was to be a 188-tonne giant fitted with a either 75mm, 128mm or even a 170mm turret mounted gun and was planned to move at a staggering 20km/h. There were two prototypes produced, going by the appropriate names of V1/"Maus I" and V2/"Maus II".
Maus I, the first turret-less prototype was completed on Christmas Eve of 1943 and was put through extensive testing. Powering its bulk was an altered Daimler-Benz MB 509 engine which could produce almost 500 horsepower, but yet was only able to move the Maus I at about 13km/h, or 5mp/h. This was not the only problem with its weight -- other weight issues were the suspension and the fact that no bridges anywhere could support its 180,000 pound mass and it had to be fitted with a snorkel so it could be driven underwater (yes, it was air-tight).
The second prototype, Maus II, was completed in mid-1944 and unlike its forebearer, included a working turret complete with 128mm and 75mm guns (which could penetrate most tank armour at over 3500 metres) and a powerplant provided by Porsche, and later replaced by another Daimler-Benz Diesel.
Upon the defeat of the Reich, several unfinished turrets and frames were discovered, and a fully assembled Maus (V2 turret on a V1 hull) is on display at the Museum of Armored Forces in Kubinka, Russia.
Source: achtungpanzer.com