Grote TG-V landship
While we all know of Hitler's fantasy super-mega-tanks, most don't realise they were not a purely Nazi conceipt but were a continuation of Soviet designs of the 1930's.
In 1930 Eduard Grote and his team from Rheinmetall were invited to the USSR, where they worked to design tanks to update the Soviet's aged armour-force. One of Grote's designs of 1931 was the massive 1000-tonne, six-turret landship. Seriously considered, the main engineering shortcoming was the lack of suitable engines to provide the 24,000 hp required to propel the vehicle at 60 kph.
Grote continued with other massive tank designs for the Soviets who were still in thrall to the concept of superheavy, multi-turret breakthrough tanks until after Barbarossa.
Grote's team left the USSR in 1933, and returned to Germany to continue work there. In 1941 Grote returned to the landship concept in Germany, reworking his earlier Stalinist 1000-tonne monster to the Hitlerite Landkreuzer P. 1000 Ratte.