At 3.36pm on December 2nd, 1942, Enrico Fermi started the nuclear age with the world's first self-sustaining chain reaction. It generated only enough power to light a single lightbulb, but this same reaction, only three years later, caused the destruction of an entire city, Hiroshima, by the atomic bomb. The initial idea, that a self-sustaining nuclear reaction might be possible, belonged to Leo Szilard, and Hungarian physicist. He envisaged there might exist an element which when bombarded by neutrons might absorb one but release two. Such an element, if gathered together in sufficiently large mass, could theoretically sustain a nuclear chain reaction. When Otto Hahn and Fritz Strassman, working in Germany, discovered uranium fission - that when uranium atoms were bombarded by neutrons they split into two - the Italian physicist, Enrico Fermi, recognised immediately the significance of their findings. Both he and Szilard, experimenting together, discovered that when uranium atoms were split additional neutron were generated which could then seed further fissions. Szilard's element had been found. More than any other person, Szilard considered the consequences of the chain reaction. Knowing war was imminent, knowing the strength of Germany's physicists, and as a Jew not under any illusions about Hitler's goals, Szilard decided to alert the US authorities to his discovery so that they, and not Germany, would succeed in building the atomic bomb first. So with the funding of the US government, Fermi continued his work. Achieving a chain reaction in natural uranium was a marginal affair and precise calculations were required. Nearly four years later, however, Fermi had designed his primitive "nuclear reactor", which he called an "atomic pile", using a pile of graphite bricks interspersed with lumps of uranium. It took over two weeks to build the pile, working in two 12-hour shifts throught the day and night. But finally, on 2nd December, on the second attempt, Fermi and his team recorded self-sustaining nuclear fission. Fermi kept up the reaction for 28 minutes so that there was no doubt about what had occurred. A year after the atomic bomb was dropped on Hiroshima and Nagasaki, Leo Szilard abandoned the study of physics.