The Lankreuzer P.1000 Ratte was a German design for a 1000 ton Tankz far heavier than the Maus.
The year is 1946 and WW2 didn't end in '45, the Ratte and other Prototypes had been built by the Germans.
The Ratte had 150–360 mm of armour
And had two 280mm Naval guns on the Turret, 1 × 12.8 cm Pak 44,
 2 × 37 mm gun, 24 × 20 mm AA gun, 24  30 mm AA gun, 8 × 88 mm AA cannon and had a crew of 41 or more, the ratte has a top speed of 40 km/h
It is not unstoppable and is a big target for bombers