The ultimate "what were they thinking"...
The Landkreuzer P-1000 "Ratte" (Rat) was an extremely large nazi pipe dream for a tank for use by Germany during World War II. It was designed in 1942 by Krupp with the approval of Adolf Hitler, but the project was canceled by Albert Speer in early 1943 and none were ever completed.
The 1000 metric ton Krupp P 1000 "Ratte", started construction but was cancelled before completion. It would have carried two 280mm guns (mounted in the same type of gun turret used in Gneisenau class warships), a single 128mm gun, eight 20mm Flak 38 anti-aircraft guns and two 15mm Mauser MG 151/15 guns.
Weighing in at 1000 metric tons (a million kilograms, or 2.2 million pounds), the P-1000 would have been over five times heavier than the Maus, the largest tank ever actually built.
It would have been equipped with six 1.2 meter tracks with three tracks per side. Its power would have come from two MAN V12Z32/44 24 cylinder Diesel marine engines with 8500hp each or eight Daimler-Benz MB501 20 cylinder Diesel marine engines with 2000hp each to achieve the 16000hp needed to move this tank.
The primary armament was a warship-like turret holding a pair of 280mm naval guns. One such turret was built before the project was canceled, and was installed in a coastal defense battery in Norway. In addition to these guns, the same type used in Gneisenau class warships, the P-1000 was to have carried a single 128mm gun, eight 20mm Flak 38 anti-aircraft guns and two 15mm Mauser MG 151/15 guns.
And yet, one colossal bad idea apparently was not enough...
An even heavier device, the P-1500 "Monster", was in the very early stages of design when both projects were canceled. It was meant to be a self-propelled version of the Schwerer Gustav (or Dora) 800mm siege cannon.
It can be noted that an actual military application of this tank would be almost impossible to find; indeed, such a vehicle would likely be unable to travel over most paved roads and bridges for risks of buckling them under its massive weight. The vehicle would additionally have been essentially incapable of dodging enemy units or avoiding land mines, and vulnerable to demolition charges and air strikes.