A plutonium sphere that was meant to be used as a core of a third atomic bomb of the same design as Fat Man to be dropped on Japan. However, after Japan's surrender, the third bomb was never assembled, and the core was later used for experiments at Los Alamos Laboratory, USA.
The core would later gain the name "Demon core" after two separate criticality accidents, each of which killed one scientist. The second incident involved Louis Slotin, a Canadian-American physicist, using a screwdriver to control the separation of two metallic half spheres (neutron reflectors) around the core. Upon the screwdriver slipping out, the half spheres closed completely and the assembly immediately became supercritical, releasing a burst of radiation, more specifically Cherenkov radiation. This procedure specifically has became a subject of several minor memes in the Japanese community, usually to suggest a character is habitually oblivious or reckless.
In 2022, a meme revolving around the Demon Core suddenly became popular outside of Japan, possibly due to Kyle Hill's and Plainly Difficult's videos covering the object. Several artists and animators have created works involving various characters replicating Slotin's experiment.
See also
Cherenkov radiation - the blue light emitted from the Demon core while going supercritical