Fat Man", an atomic fission bomb of the type dropped over Nagasaki, Japan on August 9th 1945. It was this act, along with a similar attack on Hiroshima, that forced the surrender of Japan on September 2nd of that year. The device consisted of a slightly subcritical mass of plutonium encased by explosives. The firing of these charges caused compression of the fissionable material & so rendered it supercritical. Hence this was called the 'implosion' method. The ensuing nuclear chain reaction released vast amounts of energy; despite weighing under 5 tonnes it had a yield equivalent to 20,000 tonnes of high explosive."