The bomb reproduced in a life-sized 3D image on the floor of the Imperial War Museum seems almost comical – so big and clumsy, like something out of an old film of a Jules Verne story. Surely this monster was never used. But the Soviet Union’s Tsar Bomba, the most powerful nuclear weapon ever created, was once detonated. Suspended beneath a bomber because it was too big to fit inside, it was dropped over the Barents Sea and exploded with a force of 57 megatonnes, more than 1,500 times the combined strength of the two atomic bombs America dropped on Japan.